Thin Layer Photonic Integrated Circuit Based Optical Signal Manipulators

ABSTRACT

Integrated optical intensity or phase modulators capable of very low modulation voltage, broad modulation bandwidth, low optical power loss for device insertion, and very small device size are of interest. Such modulators can be of electro-optic or electro-absorption type made of an appropriate electro-optic or electro-absorption material in particular or referred to as an active material in general. An efficient optical waveguide structure for achieving high overlapping between the optical beam mode and the active electro-active region leads to reduced modulation voltage. In an embodiment, ultra-low modulation voltage, high-frequency response, and very compact device size are enabled by a semiconductor modulator device structure, together with an active semiconductor material that is an electro-optic or electro-absorption material, that are appropriately doped with carriers to substantially lower the modulator voltage and still maintain the high frequency response. In another embodiment, an efficient optical coupling structure further enables low optical loss. Various embodiments combined enable the modulator to reach lower voltage, higher frequency, low optical loss, and more compact size than previously possible in the prior arts.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationNo. 61/832,940, filed Jun. 9, 2013; U.S. Provisional Application No.61/833,488, filed Jun. 11, 2013; and U.S. Provisional Application No.61/913,945, filed Dec. 10, 2013, the contents of which are incorporatedherein by reference in their entireties.

BACKGROUND

The present invention relates to semiconductor photonic, discrete optic,integrated optic, and opto-electronic devices. In particular, thepresent invention relates to integrated optical modulators capable ofmodulation of the light beam intensity or phase by an electrical signal.Such modulators are required for converting electrical signals intooptical signals so that the light beam can be used to transmitinformation over an optical communication system. The light source in anoptical communication system is typically a semiconductor laser and thetransmission of light is typically via an optical fiber.

The typical optical modulators available currently such as modulatorsbased on Lithium Niobate (LiNbO3) crystals, free-carriers in silicon, orsemiconductor quantum wells in compound semiconductors typically havehigh modulation voltage of around 5 Volts with around 6 dB (75%) deviceinsertion loss (meaning only 25% of optical power will go through thedevice) with modulation frequency capable of going up to 40 Giga-Hertz(40 GHz) in which 1 GHz is 10⁹ Hz. The radio-frequency (RF) powerrequired to power up an optical modulator with a modulation voltage of5V is P=V²/R in which R is typically the transmission-line impedance of50 Ohms. Thus a 5V modulation voltage will correspond to a modulator RFpower of 0.5 Watt (52/50 Watt). This power is very high especially whenthe modulator is used in an electronic-photonic integrated circuit(EPIC) or photonic integrated circuit (PIC), as the total powerconsumption of a large electronic microprocessor chip with millions oftransistor is only a few Watts. Viewing the modulator as just one activedevice, its power requirement is extremely high when compared to asingle electronic transistor, considering the fact that the typicalpower consumption of an electronic transistor in a typicalmicro-processor-type electronic chip is in the 0.2 to 1 micro-Watt (200to 1,000 nanoWatts) range per transistor at the operating speed of 1GHz. Also CMOS voltage is rapidly going below 1 Volt. Thus, modulatorcapable of operating at below 1 Volt would be of great interest to makeit compatible with CMOS circuits. While there have been efforts onreducing the modulation voltage, they are typically achieved by makingthe device length longer, which often leads to high optical loss (>75%loss). Beside the undesirable long device length, high loss and “lowvoltage” is not very useful as it would mean a higher power laser wouldbe needed to gain back the same optical power modulation and higherpower laser will consume high electrical power as well.

While there are various modulator devices with differentfunctionalities, they can share the same general device structure thatgive high device operating efficiency or device performance (e.g.modulator capable of ultra-low voltage, broad modulation bandwidth, lowoptical loss). This general device structure capable of giving highdevice efficiency or high device performance is the focus of the presentinvention.

Another area of applications is called Radio-Frequency Photonics (RFPhotonics). RF Photonics enables high frequency electrical signal to betransported via modulating an optical beam and transmit it through anoptical fiber. The electrical signal is then recovered with a high-speedoptical power detector. In RF photonics, it is also desirable to havemodulator voltage going below 1 Volt (preferably below 0.5 Volt) with alow device throughput loss of <75% for the optical beam. In all theseapplications, high modulation bandwidth of exceeding 1 Giga-Hertz's(GHz) up to tens of GHz preferably reaching 100 GHz is generallydesirable as well. In many on-chip applications, it is desirable thatthe modulator's physical length be shorter than around 1 mm. While theprior arts on optical intensity or phase modulators are able to eitherreach one or two of the desirable properties such as high modulationbandwidth (>1 GHz) or low optical-power throughput loss (<75%), they aretypically not able to reach all the desirable properties that includelow modulation voltage, high modulation bandwidth, low opticalthroughput loss, and compact device size all in one optical intensity orphase modulator device.

Modulator Figure of Merits and the Advantages of the Present Invention

As noted, achieving low modulator voltage V_(MOD) alone is notsufficient for these applications. The modulator must have low deviceinsertion loss (i.e. high optical throughput power) or high opticalpower transmissivity T defined as the modulator's output optical powerover input optical power T_(MOD)=(optical power output)/(optical powerinput). Any loss in the modulator device's optical throughput power isequivalent to low electrical signal power to optical signal powerconversion, which is an important factor to be considered in the figureof merit for the desirable modulators. As the amount of optical powermodulated is proportional to the modulator voltage, in terms of thesignal to noise transmitted, 1/T_(MOD) is equivalent to V_(MOD) ² (or1/T_(MOD) is equivalent to V_(MOD)), which is also an important factorto be considered in the figure of merit for the desirable modulators.That is, a reduction in T_(MOD) by a factor of 2 is equivalent to anincrease in V_(MOD) by a factor of 2, and is equally undesirable forachieving low signal to noise ratio in the signal modulation andtransmission. Similar arguments go to the modulation depth (MD) of themodulators defined as the percentage of the optical power being changedby the modulator. In terms of the signal to noise transmitted, 1/(MD²)is equivalent to V_(MOD) ², and shall be an important factor to beconsidered in the figure of merit for the desirable modulators.

Typically, the higher the modulator bandwidth BW (in GHz), the betterthe modulator, and is a factor to be considered in the figure of merit.Also, the shorter the modulator device length L_(MOD), the better it is.There is typically a linear tradeoff between device length and modulatorvoltage so (L_(MOD) ²) is equivalent to V_(MOD) ² as a factor to beconsidered in the figure of merit. Finally, the maximum optical power MP(in milliWatt) that can be taken in by the modulator without losingmodulation performance is also important, and is a factor to beconsidered in the figure of merit. This is some time called themodulator's optical saturation power and the modulation saturation canoccur due to carrier excitation by the high optical power passingthrough the modulator material. In term of electrical signal to opticalsignal power transfer, the higher the optical power, the more theelectrical signal can be converted back to RF voltage from the modulatedoptical beam at the photodetector end. Hence 1/(MP²) is equivalent toV_(MOD) ² as a factor to be considered in the modulator figure of merit.Bandwidth (BW) and modulation power (or energy per unit time) areinversely proportional in figure of merit. Note that 2× the modulationbandwidth means half RF energy per data transmitted as the time periodis halved. Thus signal to noise is actually worse by 2× though data rateis 2× faster—meaning the advantage of higher modulation is sort ofcancelled off by the higher noise but we will not exactly account forthe noise in the figure of merit. All faster pulses suffer the same lowpower and high noise trade off (i.e. it is not intrinsic to themodulator). We will still see larger BW as an advantage like smaller RFpower (1/V_(MOD) ²).

Combining all the above-mentioned factors, when comparing the modulatorperformances, it is useful to compare them in terms of the following“Modulator Figure of Merits” defined in terms of the quantitiesmentioned above as follows:

MFOM=T _(MOD) ²MD×MP×BW/(V _(MOD) ² ×L _(MOD) ²),  (1)

where “x” in the above equation means mathematical “multiplication” and“/” means mathematical “division”. For the discussion in this patent, wewill ignore the MP factor for simplicity (i.e. we will set MP=1). Thesquaring in V_(MOD) is because of power consideration. The other factorsare done in its equivalent to V_(MOD). Defined this way, the higher thevalue of MFOM, the better the modulator for the purpose of achievinglarge frequency bandwidth, low device's optical power throughput loss,small device size, low modulation voltage, high modulation depth, andhigh device's optical power tolerance. This MFOM shall be compared withthat of an “ideal optical intensity and phase modulator” defined below.For the purpose of applying to RF Photonics and EPICs, we define areference “ideal optical modulator” to be one that has the followingcharacteristics: T_(MOD)=0.25, V_(MOD)=0.5 (V), L_(MOD)=1=(mm), BW=40(GHz), MD=0.9. The MFOM of such an ideal modulator denoted as “MIFOM” isthen given by:

MIFOM=0.25²×0.9²×40²/(0.5²*1²)(mW²×GHz/(V²×mm²))=8(GHz/(V²×mm²)).  (2)

The ratio between the MFOM of an optical modulator and the MIFOM is ofparticular interest as it would be an indication of how close themodulator in question is to an “ideal modulator” or how much itsurpasses the ideal modulator. This ratio may be referred to as theModulator's Relative to Ideal FOM or relative modulator figure of merit(RMFOM) defined as follows:

RMFOM=(MFOM/MIFOM).  (3)

When this value is close to 1 or higher than 1, the optical modulator issaid to compare favorably with the target ideal modulator in terms ofthe modulator performances. On the other hand, when this value is muchsmaller than 1, the optical modulator is not good compared with theperformances of the target ideal modulator.

A typical performance for a “compound semiconductor based” modulator ofthe prior arts has T_(MOD)=0.25, V_(MOD)=5 (V), L_(MOD)=2 (mm), BW=40(GHz), MD=0.9. The MFOM of such a typical modulator of the prior art isgiven by:

MFOM=0.25²×0.9²×40/(5²*2²)(GHz/(V²×mm²))=0.02(GHz/(V²×mm²)).  (4)

Its RMFOM is then given by:

RMFOM=0.02/8=0.0025=(1/400).  (5)

This is 400 times worse than the targeted ideal optical modulator. Theprior arts for realizing optical modulators are thus highly inadequatein terms of realizing the targeted performances of the ideal modulator.

A typical performance for a “silicon based” modulator of the prior artshas T_(MOD)=0.25, V_(MOD)=2 (V), L_(MOD)=5 (mm), BW=10 (GHz), MD=0.9.The MFOM of such a typical silicon modulator of the prior arts is thengiven by:

MFOM=0.252×0.9²×10/(2²*5²)(mW²×GHz/(V²×mm²))=0.005(mW²×GHz/(V²×mm²)).  (6)

Its RMFOM is then given by:

RMFOM=0.005/8=0.000625=(1/1600).  (7)

This is 1600 times worse than the targeted ideal optical modulator. Theprior arts for realizing silicon based optical modulators are thushighly inadequate in terms of realizing the targeted performances of theideal modulator.

In the present invention, the major limitations of the prior arts areovercome, making it possible for the optical modulator of the presentinvention to have RMFOM ranging from 0.01 to over 2.5, which isgenerally over 10 to 1,000 times higher in term of figure of merit thanthe typical optical modulators based on the prior arts. This is a verysignificant advantage for the present invention over the opticalintensity or phase modulators based on the prior arts.

Exemplary Modulators in the Prior Art

An exemplary embodiment of the present invention for the modulationmaterial is based on utilizing the near bandgap effect of refractiveindex change due to carrier band-filling and other electro-optic effectsin compound semiconductor, resulting in electro-optic (EO) modulators.Another exemplary embodiment of the present invention for the modulationmaterial is based on utilizing the electro-absorption effects incompound semiconductor, resulting in electro-absorption (EA) modulators.

Electro-absorption modulation of an optical beam is based on opticalabsorption change due to an applied voltage, which as is well known tothose skilled in the art. Electro-absorption modulation can be inducedby a shift in the semiconductor bandgap energy under an applied voltageor induced by carrier injection-depletion that can change the opticalabsorption or gain property of the semiconductor materials. Thesemechanisms for electro-absorption modulation are relativelystraight-forward and well known in the prior art.

For electro-optic modulator, the mechanisms involved are more complex.It is thus of interest to review the physics of refractive index changein silicon-based modulators and compound semiconductor based modulatorsbelow.

Physics of Refractive Index Change in Silicon Based Modulators

The main difference between compound semiconductor based modulators andsilicon modulators for operations at the 1550 nm wavelength range forfiber-optic communications is that while silicon has an absorption bandgap, it is at 900 nm, which is far away from the operating wavelength of1550 nm. Hence, the refractive index change in silicon is not due toeffects related to the band-gap energy but due to free-carrier plasmaeffect. Free-carrier plasma can change the refractive index over a broadwavelength range but also will cause optical absorption. Hence,free-carrier plasma effect has a drawback in that when the refractiveindex change due to the free carriers is high, the amount offree-carrier absorption is also high so there is a significant amount ofoptical absorption that reduces the optical beam intensity when thephase shift induced by the free-carrier plasma is high. In silicon, therefractive index change due to the free-carrier plasma effect is knownto those skilled in the art as given approximately by:

Δn _(Plasma)=−(8.8×10⁻²² ΔN _(e)+8.85×10⁻¹⁸ ΔN _(h) ^(0.8)),  (8)

where ΔN_(e) is electron carrier density in (1/cm³), and ΔN_(h) is holecarrier density in (1/cm³). For example, with a carrier density of10¹⁸/cm³, the refractive index change of the active P-N junction regiondenoted by Δn_(Plasma) can be around Δn_(Plasma)=0.002 for holes andΔn_(Plasma)=0.0009 for electrons. These are relatively low values thatlimit the low-voltage performances of silicon-based optical modulators.

Physics of Refractive Index Change in Compound Semiconductor BasedModulator

Below, a material is said to be an active modulator material or moreprecisely an active electro-optic (EO) material if the material'srefractive index can be altered by an applied voltage, an electriccurrent, or either injection or depletion of carriers. A material issaid to be an active electro-absorption (EA) material if the material'soptical absorption or optical gain can be altered by an applied voltage,an electric current, or either injection or depletion of carriers. Theactive EO or EA materials are more generally referred to as activemodulator material or medium (ACM), or as active area, or simply asactive medium. In an active EO material, the refractive index change canthen be used to shift the optical phase of an optical beam or change theintensity of the optical beam (using for example a Mach Zehnderinterferometer as is well known to those skilled in the art). Themodulation is said to result from electro-optic modulation (as oppose toelectro-absorption modulation for which the optical absorption of themodulator material is altered). An advantage of EO modulator in generalis that it can have intensity change that is somewhat linear as afunction of the applied voltage, or can be corrected by an externalelectrical circuit to make the modulation linear. Such linear intensitymodulation capability is important for applications to Radio-FrequencyPhotonics (RF Photonics) area.

In compound semiconductor, the energy bandgap of the compoundsemiconductor, whether it is bulk compound semiconductor or quantumwells, can be designed to be close to the wavelength of operation. Forexample, in some situation, such as electro-optic modulation, it isadvantages to design the bandgap to be at 1350 nm for operation at 1550nm wavelength range. In other situation, such as electro-absorptionmodulation, it is advantages to design the bandgap to be at 1450 nm foroperation at 1550 nm wavelength range. The prominent refractive indexchange effect most frequently utilized by the prior art is quantumconfined stark effect (QCSE), which changes the refractive index due tostrong electric field applied across the quantum wells that shifts thetransition energy of the quantum wells, which then leads to a change inthe refractive index. It is purely an electric field effect and does notinvolve electron or hole carriers. The refractive index change due toQCSE is given as:

Δn _(QCSE)=(½)n ³ K _(QCSE) E ²,  (9)

where K_(QCSE)=˜0.3×10⁻¹⁸ m²/V² for InGaAsP quantum wells, n is therefractive index of the quantum well material, and E is the appliedelectric field in Volt/meter. QCSE is a quadratic electro-optic (QEO)effect as it depends on the electric field square. Another refractiveindex shift effect is due to the intrinsic electro-optic coefficient ofthe compound semiconductor material called Pockels effect. This effectis also referred to as linear electro-optic effect (LEO) of the crystal.For InP, it is given by:

Δn _(Pockels)=(½)n ³ r ₄₁ E,  (10)

where r₄₁=˜2×10⁻¹² n/V for InP material. Assuming a 2V applied voltageacross a 200 nm thick active modulator material giving an electric-fieldstrength E=1V/(100 nm)=10⁷ V/m, then the above with n=3.4 would giveΔn_(QCSE)=0.0005 and Δn_(Pockels)=0.00033. At a higher voltage of 4 V,QCSE would overtake Pockels effect substantially and becomes thedominant effect giving Δn_(QCSE)=0.002. Thus in the typical compoundsemiconductor modulator design, QCSE is the main effect used forachieving refractive index shift. Both QCSE and Pockels effect just needan applied field (i.e. no carrier is needed), which is often seen as anadvantage to achieve low optical loss (carrier can cause free-carrieroptical absorption loss).

Other effects that are seldom used in compound semiconductor basedmodulators are dependent on electron or hole carriers. One effect is thecarrier plasma effect, which is also used by silicon modulators asdiscussed above. The refractive index change due to the carrier plasma(PL) effect is given by:

Δn _(Plasma)=[(−e ²λ₀ ²)/(8π₂ c ₂∈₀ n)]*(ΔN _(d) /m _(e)).  (11)

It has a value similar to Pockels effect under the same applied field ifthe quantum well (or bulk material) doping N_(d) is n-doped with anelectron doping density of around N_(d)=N_(e)=1×10¹⁷/cm³. Another effectis due to carrier filling up the conduction bands (or valence bands). Itis called the band filling (BF) effect. FIG. 1a illustrates the case forwhich the electron carriers fill up the conduction band, leading to ashift in the absorption energy from close to the bandgap energy Eg tolarger than the bandgap energy Eg+ΔE (or in wavelength Δg−Δλ). As shownin FIG. 1b , this change in the “absorption energy edge” from absorptioncurve α_(Eg)(λ) to α_(Eg+ΔE)(λ) leads to a change in the refractiveindex of the material Δn(λ) due to what is known to those skilled in theart as the Kramer's Krognig's relation which says that a change in theabsorption spectrum Δα(λ)=α_(Eg+ΔE)(λ)−α_(Eg)(λ) as a function of thewavelength (λ) must lead to a change in the spectrum for the refractiveindex Δn(λ) as a function of the wavelength. This results in a change inthe refractive index at the operating wavelength of the beam at λ_(B)(say at 1550 nm) or its corresponding beam photon energy E_(B) due tothe absorption edge change (say at λg=1350 nm) because of carriersfilling up the semiconductor energy bands (e.g. it can be electronsfilling the conduction bands or holes filling the valence bands). Therefractive index change at the operating wavelength can be expressed as:

Δn _(BandFit) =R _(BandFit)(λ)×ΔN _(e),  (12)

where the proportionality coefficient R_(BandFit)(λ) is dependent on theoptical wavelength λ and its value would be high at close to thebandedge compared to its value at away from the bandedge. At a dopingdensity of N_(e)=10¹⁷/cm³, Δn_(BandFit) can be 2-3 times higher thanΔn_(Plasma) or Δn_(Pockels) under the same applied voltage for the III-V(e.g. InP/InGaAsP) material system when the band-edge is at 1350 nm or200 nm away from the operating wavelength of 1550 nm but may be lowerthan n_(QCSE) at a high enough applied voltage V as QCSE scales as thesquare of the electric field and is proportional to V², whereas otherchanges in the refractive index including Δn_(BandFit) are onlyproportional to V. Normally Δn_(BandFit) cannot be too high in theoptical modulator structures of the prior arts due to a few reasons.

First, Δn_(QCSE) can always be dominating at high enough voltage or highenough applied field as it is proportional to V².

Second, in the prior art, the quantum well (or bulk) active material(ACM) is either undoped or lowly doped with carriers having a dopingdensity typically lower than about 1×10¹⁷/cm³. The no doping or lowdoping is due to the problem in the modulator designs of the prior artthat high carrier doping density will substantially increase the opticalabsorption loss of a light beam going through the entire modulator. Inthe modulator design of the prior art, the optical loss would occur notonly just at the active modulator section at which the refractive indexor absorption is designed to change under an applied voltage but also inthe rest of the connecting region such as in a Mach-Zehnderinterferometer geometry. This results in a long waveguide section alldoped with the high carrier doping density, which then results in highoptical loss due to the high free-carrier density from the doping. Forexample, while the active modulator region may have a length of 0.5 mm,the entire modulator with the Mach-Zehnder interferometer geometry mayhave a total physical beam propagation length of 2-5 mm.

Third, a highly doped quantum well region will decrease what is known tothose skilled in the art as the carrier depletion width dm of the P-Njunction, a region between the P-doped region and N-doped region inwhich no or few carriers exist. The smaller the depletion width d_(PN),the larger the device capacitance as the width defines the dielectricwidth of a capacitance in which one capacitor plate is the N-dopedregion and another capacitor plate is the P-doped region, and these twoplates are separated by the depletion width d_(PN). As is known to thoseskilled in the art, the smaller the “plate separation”, the larger thecapacitance C_(PN). The large P-N junction capacitance C_(PN) willdrastically reduce the modulation bandwidth of the optical modulatorpartly due to RC frequency cutoff, where R will be the effective seriesresistance of the device R_(ser) and the RC modulator frequency cutoffwill be f_(RC)=1/(2πR_(ser)C_(PN)). Thus the quantum well in prior artis typically undoped and if it is doped, it is kept to a doping densityof 1×10¹⁷/cm³ or lower or it would be hard to achieve large modulationbandwidth for the modulator due to higher device capacitance C_(MOD)that would lead to low modulator RC cutoff frequency f_(RC), given thetypical device series resistance R_(Ser) achievable in the prior art.The 1×10¹⁷/cm³ doping level gives approximately a depletion width around100-300 nm under zero to a few volts of applied voltage, which is aboutthe active region and waveguide core thickness of a conventionalsemiconductor based modulator. Going to higher doping than 1×10¹⁷/cm³will be regarded as inefficient design as the depletion width will betoo small to cover the active region and waveguide core thickness, andalso will unnecessarily lower the modulation bandwidth due to the higherjunction capacitance. In the present invention, this limitation isovercome, resulting in substantially lower modulation voltage whilemaintaining high modulation frequency cutoff.

An Exemplary Modulator in Prior Art

FIG. 2 shows an electro-optic (EO) modulator of prior art. In particulara modulator made of III-V compound semiconductor material. Anelectro-absorption modulator in the prior art will have a similaroptical and electrical structure except that the active material isreplaced by a material that can cause electro-absorption (i.e. amaterial whose optical absorption can be altered under an appliedvoltage, an electric current, or either injection or depletion ofcarriers). For the purpose of illustration of a modulator of prior art,we will describe an EO modulator.

The EO modulator utilizes semiconductor quantum well's quantum-confinedstark effect as the main electro-optic (EO) effect. Under an appliedelectric field, the quantum well's width effectively narrows and pushesthe energy level higher, resulting in a decrease in the refractive indexdue to increase in the energy bandgap. The direction of the electricfield does not matter so it is a quadratic electro-optic effect. For1550 nm operation, a typical modulator structure is shown in FIG. 2showing device 10000. In device 10000, the device is fabricated on asemiconductor substrate SUB 10010. In an exemplary device substrate SUB10010 is N-doped InP with N-type doping density of N=3×10¹⁸/cm³. Abovethe substrate is a lower electrical Ohmic contact layer LOHC 10020. Inan exemplary device LOHC 10020 is N-doped InGaAs with N-type dopingdensity of N=1×10¹⁸/cm³ and a thickness of 0.1 μm. Above the LOHC 10020is a lower conducting waveguide cladding layer (LCWCd) 10030. In anexemplary device LCWCd 10030 is N-doped InGaAs with N-type dopingdensity of N=1×10¹⁸/cm³ and a thickness of 1.5 μm. Above the LCWCd 10030is a lower waveguide core separate confinement heterostructure (SCH)layer (LWCoSCH) 10040. In an exemplary device LWCoSCH 10040 is N-dopedInGaAlAs with energy bandgap wavelength of 1.3 μm and N-type dopingdensity of N=1×10¹⁷/cm³, a thickness of 0.1 μm. Above the LWCoSCH 10040is an active electro-optic and waveguide core layer (AEOWCo) 10050. Inan exemplary device AEOWCo 10050 is comprising of 14 quantum wells (8 nmthick) and 15 barrier layers (5 nm thick) made of InGaAlAs material withno or low doping (called intrinsic semiconductor or I-typesemiconductor), resulting in a total thickness of 0.182 μm for layerAEOWCo 10050. The quantum well layer has 35% compressive strain withrespect to InP lattice and the barrier layer has 40% tensile strain withrespect to InP lattice. Above the AEOWCo 10050 is an upper waveguideseparate confinement heterostructure (SCH) layer (UWCoSCH) 10060. In anexemplary device UWCoSCH 10060 is P-doped InGaAlAs with energy bandgapwavelength of 1.3 μm and P-type doping density of P=1×10¹⁷/cm³, athickness of 0.1 μm. Above the UWCoSCH 10060 is an upper conductingwaveguide cladding layer (UCWCd) 10070. In an exemplary device UCWCd10070 is P-doped InP with energy bandgap wavelength of 0.9 μm and P-typedoping density of P=1×10¹⁸/cm³, a thickness of 1.5 μm. Above the UCWCd10070 is a upper electrical Ohmic contact layer UOHL 10080. In anexemplary device UOHC 10080 is P-doped InGaAs with P-type doping densityof P=1×10¹⁹/cm³ and a thickness of 0.1 μm. Above the UOHC 10080 is anupper metal contact layer UMC 10090. In an exemplary device UMC 10090 issingle or multi-layer metal denoted by layer UM1 10091, UM2 10092, UM310093, . . . . with UM1 layer directly on top of UOHC 10080. In oneexemplary embodiment, UM1 is 20 nm of Ti, UM2 is 50 nm of Pt, UM3 is1000 nm of Au. On the lower side above the LOHC 10020 is a lower metalcontact layer LMC 10100. LMC 10100 is single or multi-layer metaldenoted by layer LM1 10101, LM2 10102, LM3 10103, . . . with LM1 layerdirectly on top of LOHC 10020. In one exemplary embodiment, LM1 is 17 nmof Au, LM2 is 17 nm of Ge, LM3 is 17 nm of Au, LM 4 is 17 nm of Ni, LM5is 1000 nm of Au.

Summary of Limitations of Prior Arts in Intensity or Phase Modulators

Below, we summarize further the limitations of prior arts in modulatorsby using the example of a semiconductor based modulator. The typicalsemiconductor modulators have switching voltages of around 2-5V with adevice length of a few millimeters. The optical mode is confined byweakly-guiding structure in the vertical direction with effective modesize of 0.5-1 μm and thick optical cladding of ˜1.5 μm to prevent theguided optical energy from reaching the top or bottom metal electrodewith high metal optical absorption loss. The waveguide core usually hasquantum wells (QWs) to enhance the refractive index modulation under anapplied electric field. Based on such structure, compound semiconductorEO modulators with 40 GHz modulation bandwidth have been achieved withuse of QWs and a PIN (P-doped, Intrinsic (i.e. undoped or being anIntrinsic semiconductor material), N-doped) type structure, with V□□ of2-3V (voltage that gives a total relative phase shift of □□ in themodulator Mach Zehnder Interferometer (MZI)). FIG. 3 shows the generalcross-section of such a PIN modulator structure, which is a generalschematic of the more detailed exemplary device structure shown in FIG.2. The modulator performance gives a modulator voltage of 3V, amodulation bandwidth of 30 GHz, and a modulator length of 1 mm (see [REF1]). All bandwidth noted here is analog 3 dB optical power bandwidth(not in digital Gb/s that needs smaller analog bandwidth).

There are typically four contributions to electro-optic modulation for asemiconductor modulator: (1) Linear Electro-Optic effect (LEO); (2)Quadratic Electro-Optic effect (QEO) including quantum confined starkeffect (QCSE) when quantum wells are used; (3) Band-Filling effect (BF);(4) Plasma Effect (PL). These are well described in the literature. Mostmodulators with QWs utilize LEO+QCSE as the main effects. Some mayinvolve BF and PL as additional effects.

The voltage is usually high (2-5V) in prior arts partly because theseconventional EO modulators used a weakly confined waveguide structurewith thick cladding of typically 1.5 micrometer thickness to avoid metaloptical absorption loss. This resulted in a vertical optical mode withwidth of approximately 0.5 to 1 micrometer at full-width-at-half-maximum(FWHM) power points but the full optical energy is extended to 2-3micrometers in size vertically. If the entire optical mode region isfilled with active EO materials such as quantum wells (utilizing theQCSE), then the electrode spacing will be about 2 to 3 micrometers (or2,000 nm to 3,000 nm) in spacing, which is large. The use of PINstructure can reduce the electrode spacing to say to around 200-300 nm,but then the percentage of the 200-300 nm thick electro-optic mediumoverlapping with the 1,000 nm large optical beam mode energy and the RFfield (called the mode-medium overlapping factor) will be small (down to˜10%). The applied RF electric field is usually somewhat uniformlydistributed in the active electro-optic medium. Thus, the advantage of10 times higher electric field (due to electrode spacing reduced from2,000 nm to 3,000 nm down to 200 nm to 300 nm) is cancelled by theproportionately reduced mode-medium overlapping factor (from near 100%down to ˜10%). As a result, the modulation voltage is not changed muchby the use of the PIN structure.

Due to one or more of the abovementioned reasons as illustrated via anEO modulator in the prior art, conventional semiconductor EO or EAmodulators have high modulation voltages of 2-5V depending on its devicelength and the nonlinear electro-optic effects used among the four maineffects (LEO. QCSE, BF, or PL effect). Typically 2V to 3V Vπ can beachieved with 2-3 mm long device. If all four effects are used, it couldbe reduced to ˜1 mm long device with 2V to 3V Vπ. The modulator voltagecannot be much smaller and the length cannot be much shorter and stillmaintain the high modulation bandwidth of 10 GHz or higher.

In the present invention, the above limitations of the prior arts areovercome, resulting in broadband low voltage modulators with smalldevice size having a relative figure of merits, RMFOM, that aregenerally over 10 to 1.000 times higher than modulators in the priorarts.

SUMMARY

It is an aim of the present invention to provide compact intensity orphase modulators with very low switching voltage, high modulationbandwidth, low optical loss, short modulator device length, high opticalsaturation intensity, resulting in intensity or phase modulators withexceedingly high modulator relative figure of merit that is generallyover 10 times to 1,000 times higher than the modulators based on priorarts.

It is another aim of the invention to provide compact modulator devicesthat can be used as isolated optical modulators with integrationpossibility or as isolated optical components, using either integrationtechnology or free-space and discrete optics.

The compact modulator devices can be constructed with discrete opticaland mechanical components or can be integrated in a photonic integratedcircuit or an electronic-photonic integrated circuit (EPIC).

The present invention discloses means to realize modulator devices thatwill have a wide range of utilities and can be used as devices on chipsincluding but not limited to photonic integrated circuits (PICs) orEPICs and methods of making the same.

The present invention has overcome the aforementioned limitations of theprior arts on modulators. In one embodiment of the present inventioninvolving semiconductor as the electro-active layer (ECL), thelimitation is overcome by shifting away from the use of QCSE and insteadutilizing structures that can greatly enhance the refractive indexchange based on the carrier band-filling effects by orders of magnitudelarger, the present invention achieves this by employing substantiallyhigher carrier doping density for the active material (ACM) made up ofeither quantum wells or bulk semiconductor materials.

Normally, in the prior arts, this high carrier doping density region,though may give higher modulation due to higher carrier inducedrefractive index change is intentionally avoided due to highfree-carrier absorption loss and also due to higher PN junctioncapacitance C_(PN) that can reduce the frequency response.

The present invention overcomes the disadvantages and can achieve thehigh modulation frequency and low optical loss for the device insertionin spite of the high carrier doping (and hence low modulation voltage),resulting in ultra-low-voltage high-speed low-loss ultra-compact opticalintensity and phase modulators.

For the purpose of illustrations and not limitation, these are forexample firstly achieved by utilizing “near-bandgap refractive indexchange structures” such as the band-edge band-filling effects that hashigh refractive index change (and hence lower modulation voltage) withuse of mainly the n-type carriers (that has lower optical free-carrierabsorption loss) and the use of device structure that enables only ashort length of the device that is doped (e.g. via a multilayerpassive-active device integration structure (ML-APDI)), resulting in lowtotal optical loss; secondly achieved by the employment of high-speedelectrical structures such as by lowering the resistance of thewaveguide cladding layer in the modulator using much thinner claddinglayer and by using mainly n-dopants for the electrical conduction in thecladding layer to lower the device series resistance R_(ser). The lowerR_(ser) will balance back the increased P-N or P-I-N junctioncapacitance C_(PN) due to the high carrier doping density, and hencemaintain a high RC cutoff frequency f_(RC) that is depending on theproduct of the R and C: f_(RC)=1/(2πR_(ser)C_(PN)); thirdly by using amuch thinner vertical waveguiding structure with high waveguide core tocladding refractive index ratio (also called refractive-index contrast),giving a high optical mode energy overlapping with the refractive indexchange layers (i.e. modulator active layers) or also referred to as ahigh mode-medium overlapping factor, resulting in higher change in theoptical phase shift per unit propagation distance for the optical beamand hence lower modulation voltage; and/or fourthly by eliminating thetop and bottom metal contacts that can cause optical absorption loss.The percentage of optical mode energy overlapping with the modulatoractive layers (e.g. the active EO or EA material layer) and the appliedRF field or electrical current is also called the “optical mode energyto active layer overlapping factor” or as “mode-medium overlappingfactor”. The applied RF electric field or electrical current is usuallysomewhat uniformly distributed in the active material layer.

One or more of the advantaging factors above may be employed separatelyor jointly and when more than one of these factors are applied jointly,they could mutually enhance each other. For example, the higher changein optical phase per unit propagation distance means that the devicelength can be shorter, leading also to lower total optical free-carrierabsorption loss that increases with propagating length, enabling therealization of ultra-low-voltage high-speed lowloss optical intensityand phase modulators that are also ultra-compact.

While electro-optic (EO) modulator is discussed above, several of theabovementioned advantages in the present invention can equally apply toan electro-absorption (EA) modulator by changing the active EO materialto electro-absorption (EA) material, and can use non-semiconductormaterial as the EO or EA material (i.e. as the active material (ACM)).

The modulator resulted from the present invention is capable ofachieving relative modulator figure of merits RMFOM that is 10 to 1,000times better (i.e. higher) than that of the modulators based on theprior arts. Thus, the present invention has significant performanceadvantages for integrated optical modulators over the performances ofintegrated optical modulators based on the prior arts. The fact that nosuch high RMFOM has been reached largely by the modulators in the priorarts indicate that the present invention is not presently obvious tothose skilled in the art, as in majority of the optical modulatorapplications, high RMFOM is always desirable.

An additional embodiment of the present invention is the compatibilityof the modulators with electronic-photonic (EPIC) integrated circuitplatform based on silicon-on-insulator (SOI) substrate. This embodimentis for the purpose of illustration and not limitation. For example,other types of substrate such as Gallium Arsenide (GaAs) and IndiumPhosphide (InP) can be used as long as the general structuralrequirements of the modulator in the present invention are met.

In one aspect of the present invention, the low “mode energy to activeEO material overlapping factor” or simply called the “mode-mediumoverlapping factor”, with the quantum wells in the prior art is overcomeby employing substantially thinner waveguiding layer with substantiallyhigher refractive-index contrast between the waveguide core and claddingin the vertical direction (direction perpendicular to the substrate).This is achieved, for example, by eliminating the top and bottom metalcontact enabling the thicknesses of the top and bottom cladding layersto be reduced without causing optical loss due to metal by replacing theusual metal contact with a Low-Refractive-Index Ohmic transparentconducting (LRI-OTC) material that has low optical refractive index toserve as the low-resistance electrical contact to the P-doped or N-dopedsemiconductor.

Normally, such thin waveguiding layer can increase the device insertionloss as it is hard to couple light into the layer optically, especiallyfrom an optical fiber. The present invention overcomes the disadvantageand can achieve the high device efficiency (have low power consumptionor small device size or low device optical insertion loss). The presentinvention achieves this by employing an efficient optical beam modecoupling structure to couple light between an input/output transparentwaveguide to the waveguide containing the active gain/absorption medium(called up/down coupler).

In another aspect of the present invention, the up/down coupler is haslow alignment sensitivity.

In another aspect of the present invention, the low-refractive-indexOhmic transparent conducting (TCO) material is capable of achieving“Ohmic contact” by having an electron work function reasonably matchedto a P-type or N-type semiconductor adjacent to the TCO material.

In another aspect of the present invention, the Low-Refractive-IndexOhmic transparent conducting material is a transparent conducting oxide(TCO) or large bandgap semiconductor with Fermi level reasonably matchedto a P-type or N-type semiconductor next to it so that Ohmic contact canbe achieved between the TCO and the P-type or N-type semiconductorlayer.

In as yet another aspect of the present invention, thelow-refractive-index Ohmic transparent conducting oxide for N-side Ohmiccontact is Indium Oxide (InO), Indium Tin Oxide (ITO), Zinc Oxide (ZnO),Zinc Indium Tin Oxide (ZITO), Gallium Indium Oxide (GIO), Gallium IndiumTin Oxide (GITO), and Cadmium Oxide (CdO) or materials containing anyone or more than one of these oxides.

In as yet another aspect of the present invention, thelow-refractive-index Ohmic transparent conducting oxide for P-side Ohmiccontact is Indium Oxide (InO), Indium Tin Oxide (ITO). Zinc Oxide (ZnO),Zinc Indium Tin Oxide (ZITO), Gallium Indium Oxide (GIO), Gallium IndiumTin Oxide (GITO), and Cadmium Oxide (CdO), or materials containing anyone or more than one of these oxides.

In as yet another aspect of the present invention, the low mode-mediumoverlapping factor of the prior art with the quantum wells is overcomeby using a side conduction structure for the top metal contact to thetop P-doped or N-doped semiconductor.

In another aspect of the present invention, the difference between therefractive index in the waveguide cladding and refractive index in thewaveguide core is in the very-strongly to medium-strongly waveguidingregion for waveguide confinement in the direction perpendicular to thesubstrate.

In another aspect of the present invention, the difference between therefractive index in the waveguide cladding and refractive index in thewaveguide core is in the weakly waveguiding region for waveguideconfinement in the direction perpendicular to the substrate.

In another aspect of the present invention, the thickness of the centralwaveguide core in the active layer structure (ALS) called theelectro-active waveguiding core is either in the ultra-thin or very-thinregime.

In another aspect of the present invention, the thickness of the centralwaveguide core in the active layer structure (ALS) called theelectro-active waveguiding core is either in the ultra-thin, very-thin,or medium-thin regime.

In another aspect of the present invention, the thickness of the centralwaveguide core in the active layer structure (ALS) called theelectro-active waveguiding core is in the thin regime.

In another aspect of the present invention, the electro-activewaveguiding structure shall be in the very-strongly guiding regime, andthe thickness of the electro-active waveguiding core shall either be inthe ultra-thin regime or very-thin regime.

In another aspect of the present invention, the electro-activewaveguiding core structure shall be in the medium-strongly guiding orvery-strongly guiding regime, and the thickness of the electro-activewaveguiding core shall either be in the ultra-thin regime, very-thinregime, or medium-thin regime.

In another aspect of the present invention, the electro-activewaveguiding core structure shall be in the weakly guiding regime, andthe thickness of the electro-active waveguiding core shall either be inthe ultra-thin, very-thin, medium-thin, or thin regime.

In as yet another aspect of the present invention, the contact made bythe low-refractive-index Ohmic transparent conducting material ispreferably to N-doped semiconductor at both top and bottom part of thedevice structure, as N-doped semiconductor has lower electricalresistance than P-doped semiconductor and also has low opticalabsorption loss than P-doped semiconductor.

In as yet another aspect of the present invention, only one of the topand bottom contacts employs contact via low-refractive-index Ohmictransparent conducting material.

In as yet another aspect of the present invention, a pair of doped thinP—N layers serves as a hole-to-electron PN-changing PN junction or PNtunnel junction so as to change the metal contact with P-dopedsemiconductor to a metal contact with N-doped semiconductor, whichenables drastic reduction in the Ohmic contact resistance as N-dopedsemiconductor is easier to achieve low Ohmic contact resistance.

In as yet another aspect of the present invention, the low mode-mediumoverlapping factor of the prior art with the quantum wells is overcomeby using a side conduction structure for the top metal contact to thetop P-doped or N-doped semiconductor. The side conduction geometryenables the waveguiding layer to be thin giving high mode-mediumoverlapping factor and yet maintaining low optical loss as the opticalbeam energy will not touch the optically lossy metal that is alreadymoved to the side. Often the top cladding in such thin waveguidestructure can be made to be either air or some low-refractive-indexdielectric material. In as yet another aspect of the present invention,the metal is deposited on both sides of the contact with as large anarea as possible to reduce the Ohmic contact resistance. The sideconducting layer is highly doped to reduce side conduction resistancebut not so highly doped as to cause excessive optical absorption lossdue to free carriers.

In as yet another aspect of the present invention, the top sideconduction structure is preferably N-doped semiconductor with lowerelectrical resistance than P-doped semiconductor and also has lowoptical absorption loss than P-doped semiconductor.

In as yet another aspect of the present invention, the low mode-mediumoverlapping factor of the prior art with the quantum wells is overcomeby using a side conduction structure for the bottom metal contact to thebottom P-doped or N-doped semiconductor. The side conduction geometryenables the waveguiding layer to be thin giving high mode-mediumoverlapping factor and yet maintaining low optical loss as the opticalbeam energy will not touch the optically lossy metal that is alreadymoved to the side. Often the bottom cladding in such thin waveguidestructure can be made to be either air or some low-refractive-indexdielectric material.

In as yet another aspect of the present invention, the bottom sideconduction structure is preferably N-doped semiconductor with lowerelectrical resistance than P-doped semiconductor and also has lowoptical absorption loss than P-doped semiconductor.

In as yet another aspect of the present invention, both the top andbottom side conduction structure is preferably N-doped semiconductorwith lower electrical resistance than P-doped semiconductor and also haslow optical absorption loss than P-doped semiconductor.

In as yet another aspect of the present invention, only one of the topand bottom contacts employs side conduction structure.

In as yet another aspect of the present invention, the side conductionstructure includes a top transparent dielectric region over which and onboth sides are deposited with metal for mechanical robustness.

In as yet another aspect of the present invention, a top transparentdielectric region acts as a lateral confining rib waveguiding structure.

In as yet another aspect of the present invention, a bottom transparentdielectric region acts as a lateral confining rib waveguiding structure.

In as yet another aspect of the present invention, a center dielectricregion acts as a lateral confining rib waveguiding structure.

In as yet another aspect of the present invention, a wide top contactregion acts to reduce the device contact resistance and conductionresistance, and together with the top metal side contact structure, actsto increase the optical mode energy overlapping with the active medium.

In as yet another aspect of the present invention, a waveguidingstructure acts to propagate the optical beam into the modulatorstructure and avoid the extension of the highly doped quantum wells orbulk material to the transparent waveguiding region, thereby reducingthe beam propagation length through the quantum wells or bulk materialregion that has high optical absorption. This reduces the total opticalbeam absorption loss through the modulator device.

In as yet another aspect of the present invention, the waveguidingstructure tapers down to a width smaller than a wavelength in thewaveguiding material so as to strongly push the mode away from the lowertransparent waveguiding structure towards the quantum wells or bulkmaterial, thereby increasing the optical mode energy overlapping withthe quantum wells or bulk material.

In another aspect of the present invention, the refractive index changein the beam area required to achieve optical modulation is altered bydepleting carrier from or injecting carrier to the N-doped or P-dopedactive region and alter the optical transition energy edge. This leadsto a change in the smallest energy in which optical transition can takeplace. This change in the smallest energy in which optical transitioncan take place alters the refractive index of the medium.

In one aspect of the present invention, this carrier depletion orinjection is from or to one or more quantum wells in the waveguidingbeam energy area.

In one aspect of the present invention, this carrier depletion orinjection is from or to bulk semiconductor material in the waveguidingbeam energy area.

In another aspect of the present invention, the active medium is made upof compound semiconductor in which the material composition is chosen toresult in an energy bandgap reasonably close to the optical wavelengthof operation to result in high enough refractive index change due toband filling.

In another aspect of the present invention, the energy bandgap of theactive medium is less than 30% of the photon energy away from the photonenergy of operation to result in high enough refractive index change.

In as yet another aspect of the present invention, the active modulatoractive material (ACM) can be formed by non III-V materials such assilicon-germanium (SiGe) quantum wells or SiGe bulk materials.

In as yet another aspect of the present invention, the active medium(ACM) has high-level doped carrier density with P-type or N-type dopingand a doping density higher than 2×10¹⁷/cm³ and lower than 5×10¹⁷/cm³primarily but not exclusively for low modulation voltage V_(MOD) or lowmodulation RF power P_(MOD) (low means V_(MOD)<1.75 Volt or P_(MOD)<60mW), and low-loss high-frequency modulator applications. TypicallyV_(MOD) and P_(MOD) are approximately related by the R_(LOAD)transmission line resistance or load resistance: P_(MOD)=V_(MOD)²/R_(LOAD). In an exemplary embodiment, R_(LOAD) is around 50 Ohms.

In as yet another aspect of the present invention, the quantum wellshave medium-high-level doped carrier density with P-type or N-typedoping and a doping density higher than or equal to 5×10¹⁷/cm³ and lowerthan 1.5×10¹⁸/cm³ primarily but not exclusively for medium-lowmodulation voltage V_(MOD) or medium-low modulation RF powerP_(MOD)(medium-low means V_(MOD)<1 Volt or P_(MOD)<20 mW), and low-losshigh-frequency modulator applications.

In as yet another aspect of the present invention, the quantum wellshave very-high-level doped carrier density with P-type or N-type dopingand a doping density higher than or equal to 1.5×10¹⁸/cm³ and lower than5×10¹⁸/cm³ primarily but not exclusively for very-low modulation voltageV_(MOD) or very-low modulation RF power P_(MOD) (low means V_(MOD)<0.6Volt or P_(RF)<7 mW), and low-loss high-frequency modulatorapplications.

In as yet another aspect of the present invention, the quantum wellshave ultra-high-level doped carrier density with P-type or N-type dopingand a doping density higher than or equal to 5×10¹⁸/cm³ primarily butnot exclusively for ultra-low modulation voltage V_(MOD) or ultra-lowmodulation RF power P_(MOD) (ultra-low means V_(MOD)<0.3 Volt orP_(MOD)<2 mW), and low-loss high-frequency modulator applications.

In as yet another aspect of the present invention, the quantum wells canbe strained, unstrained, double-well, or multi-well quantum wells.

As an exemplary embodiment, the low voltage/power case can achieve aRMFOM>0.05 and the ultra-low voltage/power case can achieve a RMFOM>2.5,while the prior art can only achieve a RMFOM<0.005.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the invention will hereinafter be describedin conjunction with the appended drawings provided to illustrate and notto limit the invention, wherein like designations denote like elements,and in which:

FIG. 1a is a diagram illustrating the case for which the electroncarriers fill up the conduction band, leading to a shift in theabsorption energy from close to the bandgap energy Eg to larger than thebandgap energy Eg+ΔE (or in wavelength λγ−Δλ);

FIG. 1b is a diagram showing a change in the “absorption energy edge”from absorption curve α_(Eg)(λ) to α_(Eg+ΔE)(λ) leads to a change in therefractive index of the material Δn(λ).

FIG. 2 is a diagram showing an electro-optic (EO) modulator of priorart. In particular a modulator made of III-V compound semiconductormaterial.

FIG. 3 is a diagram illustrating the cross-section of the PIN modulatorstructure.

FIG. 4a is a diagram showing the location of the “modulator input beamcoupler structure (IBCS)”, the “Active Layer structure (ALS)”, and the“modulator output beam coupler structure (OBCS)”

FIG. 4b is an exemplary schematic for a cross-section of the inputwaveguide region. The region occupied by the optical beam (OB) is shownas a shaded region.

FIG. 4c is an exemplary schematic for a cross-section of IBCS region orOBCS region. The region occupied by the optical beam (OB) is shown as ashaded region.

FIG. 4d is an exemplary schematic for a cross-section of ALS regionshowing the active material (ACM) layer, the electro-active layer (ECL),and the waveguiding layers. The region occupied by the optical beam (OB)is shown as a shaded region.

FIG. 5a is a diagram showing a pair of capacitively-loaded travelingwave electrodes.

FIG. 5b is a diagram showing the cross-section d-d′ in FIG. 5a ,illustrating that the bottom parts of the two phase modulators areconnected.

FIG. 6a is a diagram showing equivalent lumped-element circuit of a pairof capacitively-loaded traveling wave electrodes (CL-TWE) powering thetwo phase modulators along the two arm of the Mach ZehnderInterferometer.

FIG. 6b is a diagram showing the top view of the CL-TWE lines for thetraveling wave electrodes.

FIG. 7 a/b is a diagram illustrating how the beam from the SOI waveguideis coupled into the thin-film modulator structure using two tapers

FIG. 8a is a diagram showing the cross-section for theside-conduction-layer (SCL) case.

FIG. 8b is a diagram showing the cross-section for the Ohmic TransparentConductor (OmTC) case.

FIG. 9 is a diagram showing the details structures for an exemplary NINmodulator.

FIG. 10 is a diagram showing the details structures for an exemplaryNPNN modulator.

FIG. 11 is a diagram showing the modulation BW for the NIN case with2-μm-wide active region so w_(CAP)=2 μm.

FIG. 12 shows the modulation bandwidth for the NPNN case with 2-μm-wideactive region so w_(CAP)=2 μm.

FIG. 13 is a diagram illustrating a general geometry of the EO Modulatorof the present invention. FIG. 13b is a semi-transparent illustration ofFIG. 13 a.

FIG. 14 shows a diagram illustrating that material surrounding the inputconnecting waveguide core.

FIG. 15 shows a diagram illustrating the input beam coupler structure(IBCS). FIG. 15a show the input beam coupler structure (IBCS) comprisesat least a tapering waveguide section generally tapering from wide tonarrow. Optionally, the active layer structure ALS on top of the inputtapering waveguide section can also be tapering in the form of an uptaper (tapering from narrow to wide in the direction toward the activelayer structure ALS). See for example FIG. 15 b.

FIG. 16 shows a diagram illustrating a few exemplary cases for the topmetal contact pad placements.

FIG. 17 shows a diagram illustrating a particular exemplary embodimentof a electro-active waveguiding core structure EWCoS 22600.

FIG. 18 shows a diagram illustrating the output connecting waveguidestructure.

FIG. 19 shows a diagram illustrating the division of the outputconnecting waveguide cladding into four different regions.

FIG. 20 shows a diagram illustrating the output beam coupler structure(OBCS)

FIG. 21 shows a diagram illustrating the electro-active layer (ECL).

FIG. 22a shows a diagram illustrating the structure around the PqN. NqN,or PN junction.

FIG. 22b shows a diagram illustrating the structure around the PqN, NqN,or PN junction with PN changing PN junction.

FIG. 23 shows a diagram illustrating an alternative structure involvedhaving the metal Ohmic contact on the side away from the center regionof layer TVSCOC 21700.

FIG. 24 shows a diagram illustrating an alternative structure involvedhaving the “top lateral conduction geometry with metal contact” but alsoa lowloss dielectric material as layer 21810.

FIG. 25 shows a diagram illustrating an alternative structure for whichthe metal can even go on top of the top lowloss dielectric materialregion TDMR 21810 to make this top lateral conduction structuremechanically robust.

FIG. 26 shows a diagram illustrating an alternative structure with lowcapacitance but also low optical loss for example by using the topvertical/side conduction and Ohmic contact layer TVSCOC 21700 to confinethe mode laterally.

Skilled artisans will appreciate that the elements in the figures areillustrated for simplicity and clarity, and have not necessarily beendrawn to scale. For example, the dimensions of some of the elements inthe figures may be exaggerated, relative to the other elements, to helpin improving understanding of the embodiments of the present invention.

DETAILED DESCRIPTION Motivations of the Present Invention

There are various needs for ultra-low-RF-power ultra-wide-RF-bandwidthlow-optical-loss high-optical-power modulators for various applications.Certain exemplary modulators employing exemplary embodiments of thepresent invention are capable of either Ultra Low Voltage, Ultra-HighModulation Bandwidth, Low Optical Loss, or High Optical Power, or aplurality of the above. In addition, they are generally ultra-compact,can be integrated with semiconductor laser, and can be made based onmass-producible silicon-photonic platform with EPIC (electronic-photonicintegrated circuit) capability enabling future expansions to integratewith RF circuits or other photonic devices on chip.

Needs for Compact Wide-Bandwidth Low-Power Low-Loss Modulators

New applications in communications and sensing require transmission ofhigh-frequency electronic signals. Transmission of ultra-fast digitaldata over optical fiber system is also important for next generationdata centers. In order to address such needs, optical phase or intensitymodulators that are capable of low switching voltage (lower than 0.5 to1V), broad RF bandwidth (BW) (higher than 20 GHz; preferably over 100GHz), low device optical power loss (preferably <6 dB), and capable ofwithstanding optical powers of a few hundred milliWatts would bedesirable. Prior arts in EO modulators are not able to realize suchmodulators. For example, the commonly available Lithium Niobatemodulators in the market can go up to 40-100 GHz but cannot reach lowenough modulation voltage of below 1V. Polymer modulators could givebroader bandwidth of over 100 GHz but still have high switchingvoltages. Also they cannot withstand high optical powers. Semiconductorbased modulators have various advantages in terms of their smaller sizeand shorter physical length but they are not yet able to give modulationvoltage lower than 1V, broad modulation bandwidth higher than 20 GHz,and low device optical throughput loss smaller than 6 dB concurrently.

Exemplary embodiments of the present invention described below willutilize a few key factors combined to fully address the abovementionedproblems resulting in low modulation voltage and wide modulationbandwidth concurrently, In some preferred embodiments, the resultingmodulator also has short physical device length and low optical loss.

Broad Overview of the Present Invention

An optical modulator device can be divided into a few key componentscomposing the modulator. An input optical beam must be channeled to themodulator's active material medium (ACM) efficiently without too muchloss of the beam's optical power. In conventional EO modulator, this isjust done by joining input waveguide to the modulator waveguide. In theEO modulator of the present invention, due to the small optical mode inthe modulator, in one exemplary embodiment, this input structure is anintegral part of the modulator of the present invention. We call thisthe “modulator input beam coupler structure (IBCS)”. A diagramillustrating such IBCS is shown in FIG. 4. An exemplary schematic for across-section of the input waveguide region is shown by FIG. 4b . Anexemplary schematic for a cross-section of such IBCS region is shown byFIG. 4c . The region occupied by the optical beam (OB) in FIG. 4b andFIG. 4c is shown as a shaded region.

The modulator input beam coupler structure brings the optical beam froma waveguide into the modulator's main waveguide that contains the activeelectro-optic (EO) material or electro-absorption (EA) material. Anactive EO (or EA) material is a material whose refractive index (orabsorption or gain) can be altered by an applied electric field or anelectrical current. Such EO or EA material will be called collectivelyas active material or medium (ACM). The active medium is typicallyembedded as part of the active layer stricture (ALS) of the modulator.An exemplary schematic for such ALS is shown by FIG. 4d . The activemedium (ACM) plus the immediately connected structure next to it forapplying the required electric field or introducing the required currentis called the electro-active layer (ECL). An exemplary schematic forsuch ECL is shown by FIG. 4 d.

In addition, a more extensive electrical conduction structure isintegrated with the electro-active layer and a waveguiding structure sothat an electric field or electrical current can be brought into theelectro-active layer encompassing the active material, and at the sametime the waveguiding structure will guide an optical beam so that partof its beam power overlaps with the EO/EA material. This enables theoptical beam to experience the change in phase shift induced by a changein the refractive index in the EO material layer or a change in theoptical absorption (or in some cases optical gain) in the EA materiallayer.

A most commonly used structure immediately connected to and next to theactive material for applying the required electric field or introducingthe required current is a PIN structure, meaning that the electricalconduction to the active material is with a P-doped semiconductorfollowed by an intrinsic (“I”) semiconductor, and then followed byanother N-doped semiconductor. An exemplary schematic for such PINstructure is shown by FIG. 4d . The active medium is typically in the Ilayer but can also be in any of the P or N layer, including thetransition region between the I and any of the P and N layer, or inplurality of these layers. The active medium can be the layer itself, ora quantum-well structure, or other active-medium structure embedded inthe layer. Most commonly, the active material is quantum wells in theintrinsic layer. Most EO and EA modulators with such PIN structure andquantum wells in the intrinsic layer as the electro-active layeroperates under a reverse bias to such a PIN structure (with negativevoltage applied to P side and positive voltage applied to the N side).Reverse bias will not bring in much current to the active material butwill bring in a strong electric field to the active material. Theelectric field acting on the quantum well then changes its refractiveindex or absorption at the beam's optical wavelength. In some situation,a modulator can operate with gain and a forward bias can be applied tosuch PIN structure (with positive voltage applied to P side and negativevoltage applied to the N side). Forward bias brings a strong injectioncurrent to the active material. The electrical current exciting thecarriers in the quantum well then changes its refractive index or gain(or absorption) at the beam's optical wavelength. Typically, modulatorbased on forward bias has a slow turning off characteristics due to theslow nanosecond decay of excited carriers in the active medium after thevoltage is turned off. Hence, typically the reverse bias case will giverise to much faster modulation speed than the forward bias case.

For modulators with, for example PIN junction or the like, theElectro-Active layer (ECL) will be the EO/EA material and the immediateP and N doped regions or the like. An exemplary schematic for such ECLwith a PIN structure and the EO/EA active material (ACM) layer is shownby FIG. 4 d.

The entire larger structure is called the active layer structure (orALS) below. In short, the entire structure comprising: (1) thewaveguiding layers: (2) the electro-Active layers; and (3) the otherelectrical conduction layers, is called the active layer structure (ALS)of the modulator in the present invention. An exemplary schematic forsuch ALS is shown by FIG. 4d . In many situations, part of theElectro-Active layer or part of the other electrical conduction layersalso serve the double function as part of the waveguiding layers. Thus,each these layers often by itself, serves as multiple-function layer.

The optical waveguide in the ALS is called active waveguide, so as todistinguish it from the input and output waveguides that have no activeEO/EA material. In another exemplary embodiment, the present inventionis concerning on the specific structure of the active layer structureindependent of the input and output mode coupling structures. Thelocation of such ALS layer is shown in FIG. 4.

At the output, we have a “modulator output beam coupler structure(OBCS)” that couples the optical beam efficiently from the modulatoractive layer structure into a primarily passive output opticalwaveguide. Passive in this context means the waveguide acts primarily totransmit the optical beam energy. Primarily passive means it can also beactive (e.g. with optical gain, absorption, or modulation) but for thepurpose of this invention, the passive beam transmission function is thefunction utilized. In as yet another exemplary embodiment, this outputstructure is an integral part of the modulator of the present invention.The location of such OBCS is shown in FIG. 4. The cross section of OBCSis similar to that of IBCS, An exemplary schematic for a cross-sectionof such OBCS region is shown by FIG. 4c . The region occupied by theoptical beam (OB) is shown as a shaded region.

For the purpose of illustration but not limitation, it is useful toprovide an overview of an exemplary modulator device employing thepresent invention. The Active-Layer Structure in an exemplary EOmodulator of the present invention could make use of up to six main keyelements, namely: (1) The use of an efficient coupling waveguideplatform (EC-WG); (2) Low-Optical-Loss Ohmic Contact (LOL-OC), such asthe use of transparent conductor and side conduction geometry, (3)Low-Optical-Loss and High Electrical Conductivity Waveguide Structure(LOL-HEC-WS) such as the use of PN-changing PN junction or PN tunneljunction to reduce the region with P-doping, (4) High-Response ActiveMaterial, such as material that has high EO or EA response under appliedvoltage, an electric current, or either injection or depletion ofcarriers (HR-AM); in an exemplary embodiment of the present invention,this is achieved with appropriately high carrier doping in quantumwells, and (5) Highly-Confined Thin-Film Electro-Active Waveguide(TF-ECW), so as to increase the amount of overlapping between theoptical mode energy and the active material.

To summarize these few advantages, the modulators of the presentinvention encompass one or more of the above five main key elements,including the advantages in Beam input/output Coupling Waveguide, OhmicContact, Waveguide Conductive Structure, Active Layer, and StronglyConfined Thin-Film Active Waveguide. For general references,optoelectronic or photonic device structures that take advantage of afew of the above five main factors will be generally referred to asWOCAT device structures. While the WOCAT structure here is applied tooptical phase and intensity modulators, it has applications beyondoptical modulators such as applications to optical amplifier,photodetector, laser, light-emitting device, optical switching and logicdevice, and optical signal processing device.

As an exemplary embodiment, for the purpose of illustration and notlimitation, the EO Modulators of the present invention are capable ofachieving the significant advantages of ultra-low-voltage (<0.5Vtypical), broad RF bandwidth (20-100 GHz), low optical loss (<6 dB),short device length (<1 mm), or high optical power (>100 mW), or aplurality of the above. As an exemplary illustration but not limitation,such a modulator could make use of one or more of the following few keyfactors in its structure.

Key Factor I: Low Voltage Via Strong Mode Confinement

For the purpose of discussion and not limitation, for an operatingoptical wavelength of 1550 nm, an exemplary approach will be based onInP/InAlGaAs material system (called simply as InP/III-V). When used as1550 nm EO modulation material, the InP/III-V material system willinvolve quantum wells (QWs) at 1300-1400 nm wavelength range that isclose enough to the 1550 nm operation wavelength to give high EO phaseshift, but is still far away from 1550 nm so that optical carrierexcitation will be low enabling high optical power (>100 mW). This willresult in a high maximum optical power or high MP value (used in theModulator Figure of Merits: MFOM). When used as 1550 nm EA modulationmaterial, the InP/III-V material system will involve quantum wells (QWs)at 1400-1550 nm wavelength range that is close enough to the 1550 nmoperation wavelength to give high electro-absorption, and in some cases,optical gain (both absorption and gain can be used to modulate theintensity of an optical beam). However in that case, the MP value willbe smaller due to the optical beam energy being absorbed and saturatingthe absorbing quantum well, thereby reducing the amount of opticalmodulation due to electro-absorption, when the optical beam power ishigh.

In order to achieve very low modulation voltage, one way is to make theoptical mode confinement a lot tighter so as to drastically reduce theeffective distance between the voltage-applying electrodes. This willenable the same electric field strength to be achieved withproportionally lower applied voltage (assuming the mode-mediumoverlapping factor is near 100% and cannot be increased further). If themode-medium overlapping factor is small, then a stronger modeconfinement giving a small mode size will increase the overlapping,which will also increase the modulation phase shift or absorption evenwith the same electric field strength. In either case, the voltage islower due either to higher electric field or higher mode-medium overlap.

For a conventional semiconductor EO or EA modulator, the vertical modesize (FWHM) is about 0.5 μm to 1 μm. With high-refractive-index-contractmaterial using semiconductor with high refractive index (n˜3 to 4) asthe waveguide core surrounded by air, dielectric material, or polymerwith low refractive index of n˜1 to 2 as waveguide cladding, it ispossible to reduce the vertical mode size by about 2.5 times to 10 timesto ˜0.1-0.2 μm (at λ=1550 nm). For example, using the high refractiveindex of III-V semiconductor with n˜3.5 as the waveguide core and air ascladding will result in a single-mode strongly confined waveguidephysical height of about 0.2 μm, given by d_(SM)˜λ/(2n)=1500nm/(2*3.5)=0.214 μm. This gives a vertical mode size of ˜0.1 μm, whichis a 5 to 10 times reduction in mode size or 5 to 10 times increase inthe mode-medium overlapping factor. Suppose other factors remain thesame, the modulation voltage is inversely proportional to themode-medium overlapping factor. Thus, the 5 to 10 times highermode-medium overlapping factor will reduce the modulation voltage by 5to 10 times, and the 5V modulation voltage of a typical conventionalmodulator structure can be reduced to below 1 volt with use of themodulator structure of the present invention that has a strong modeconfinement.

For the purpose of illustrations and not limitations, unless otherwisestated, all the dimensional numbers such as mode size and structuralsizes given in this invention assume that the operating opticalwavelength is around the wavelength of 1550 nm. As is well known tothose skilled in the arts, all these dimensions will scaleproportionally when operating at other wavelength so that if theoperating wavelength is at around 750 nm, all the physical dimensionswill be about half of that given for the 1550 nm wavelength case. Thisinvention is applicable to all other wavelengths and is not limited tothe exemplary operating wavelength of 1550 nm.

Key Factor II: Efficient Coupling into Strongly-Confined Waveguide

In the present invention, the vertical mode confinement is reduced to<0.2-0.3 μm. A challenge is how to achieve efficient optical beamcoupling to the sub-micrometer waveguide. We will solve this problemusing tapered waveguide that can be fabricated on a substrate and theALS thin film on top of it can be attached via wafer bonding method orother methods after the waveguide is fabricated. Such tapered waveguidecoupling structure can achieve over 90-95% optical power couplingefficiency.

Key Factor I: High Modulation Bandwidth Via Low Ohmic Contact Resistance& Low-Optical-Loss High-Conductivity Waveguide Structure; and HighModulation Bandwidth Via Enhanced EO Response

To enable electrical current injection and voltage conduction into themodulator device with strong optical mode confinement, as an exemplaryembodiment in the present invention, a transparent conducting (TC)material that has low refractive index and yet can achieve excellentOhmic contact with N-doped InP semiconductor material with very lowcontact resistance is used. We call these Ohmic Transparent Conducting(OmTC) materials. These TC materials are typically metal oxides (In₂O₃,ZnO, InSnO, CdO, ZnInSnO, InGaO, etc) or doped metal oxides (e.g. theabove listed metal oxides doped with magnesium Mg or zinc Zn etc), themost familiar one is ITO (Indium Tin Oxide; InSnO) used widely in LCDdisplay. They are called transparent conducting oxides (TCO). Forexample, with appropriate processes, it is possible to achieve goodOhmic contact between In₂O₃ or CdO and N-doped InP. We will call theseOhmic TCO (OmTCO). OmTCO will enable robust electrical structures to berealized that also has high mode confinement.

Alternatively, we can use a side conduction layer (SCL) to bring thevoltage into the top layer for the low voltage modulators. The sideconduction geometry enables the waveguiding layer to be thin giving highmode-medium overlapping factor and yet maintaining low optical loss asthe optical beam energy will not touch the optically lossy metal that isalready moved to the side. Often the top cladding in such thin waveguidestructure can be made to be either air or some low-refractive-indexdielectric material.

Both OmTCO or SCL can be used for the top contact. When the structure isthin, comparing to SCL the use of OmTCO for top contact has an advantagein terms of ease in fabrication and also potentially better deviceperformances as the metal contact area can be larger.

However, solving electrical conduction is only half of the matter. Themodulation speed of a conventional semiconductor EO or EA modulator isabout 20-40 GHz. It is desirable to reach 100 GHz with voltage <1V. Howcan one do so? It turns out that the high contact resistance for thep-doped material with metal, that is the high P-Ohmic contact resistancewith metal, is a main problem that limits higher modulation frequency.P-Ohmic contact typically has 10 times higher resistance than N-Ohmiccontact (with their respective appropriate Ohmic-contact metals that cangive reasonably low contact resistance).

We note that at the same dopant density, the P-doped cladding layertypically also has about 10 times higher resistance than if it isN-doped. While the cladding resistance is typically smaller than theP-Ohmic contact resistance especially since the active-layer structureis thin, the P-doped cladding can cause radio-frequency (RF) loss when ahigh-frequency modulating voltage pulse (or electrical signal)propagates along the modulator structure. In terms of free-carrieroptical absorption, at the same dopant density, the P-doped claddinglayer is also about 10 times higher than N-doped. If one reduces theP-doped cladding resistance by increasing the carrier doping density,one will also increase the optical loss, making it hard to achieve thelong modulator length needed for low voltage, giving a trade-off betweenlow voltage (need low doping density) and high frequency (need highdoping density).

To gain higher electrical modulating frequency response for themodulator, it is important to reduce the P-Ohmic contact and claddingresistance by changing them to N-contact and N-doped cladding instead. Afew exemplary structures can do so. These structures are broadlyclassified as alternative Electro-Active Layer Structure A, B, C, and Ddiscussed below.

Alternative Electro-Active Layer Structure A: NIN Structure

Besides the usual PIN structure noted above that can be used as theelectro-active layer structure in the ALS, there are other alternativeelectro-active layer in the ALS structures that may have certainadvantages. First is the use of an NIN electro-active layer in the ALSstructure, meaning that the electrical conduction to the active mediumis with an N-doped semiconductor followed by an intrinsic (“I”)semiconductor, and then followed by another N-doped semiconductor. Theactive medium is typically in the I layer but can also be in any of thetwo N layers, including the transition region between the I and any ofthe two N layers, or in plurality of these layers. The active medium canbe the layer itself, or a quantum-well structure, or other active-mediumstructure embedded in the electro-active layer.

In the situation whereby the modulator devices in the present inventionrequires largely only electric field to be applied to the active mediumto affect the refractive index or optical absorption (or optical gain)of the active medium, it is appropriate to use such a NIN structure asthe active layer.

Comparing to the use of the conventional PIN structure, such NINstructure will reduce the Ohmic contact resistance by 10 times as bothsides of the metal contacts will be contacting to N-doped layers only.Note that in NIN, sometime a thin P-doped layer is introduced so that itforms NPIN, where the P-layer helps to block the electric current, the“NIN” here broadly includes NPIN. For NPIN case, a positive voltageapplied to the N layer of the NP side will cause reverse bias at the NPjunction and hence cut off any current flow (PIN side becomes forwardbias). A positive voltage applied to the N layer of the PIN side willcause reverse bias at the PIN junction and hence also cut off anycurrent flow. This reverse bias to the PIN junction case is normallypreferred as it will mimic the revered bias to the conventional PINstructure case more closely with voltage drop mainly across the PIN partof the structure (instead of the NP part of the structure).

The NIN structure also reduces the RF-loss due to P-doped cladding by 5to 10 times. This enables the maximum length of the modulator limited byRF-loss cutoff to reach a longer length of over 1 cm (for 40 GHzbandwidth). RF-loss is sensitive to the capacitance loading of themodulator. The capacitance is mainly determined by the lateral width ofthe “junction capacitance region”, called modulator-capacitor lateralwidth (labeled as w_(CAP) or w_(EC)) below. Lateral is in a directionperpendicular to the direction of optical beam propagation and parallelto the substrate surface.

For a modulator with a typical ˜200 nm thick active region that definesthe “effective separation of the junction capacitor plates”, therequired modulator capacitor lateral width has to be w_(EC)<2-μm inorder to reach ˜40 GHz BW for a 1 cm-long modulator. At 100 GHz, thisRF-loss limited modulator length could be around 3-5 mm (for the same2-μm modulator capacitor width).

The optical-absorption-loss limited length (at 2 dB attenuation so as tokeep the total loss less than 6 dB) due to N-doped claddings in NIN canbe made to reach longer than 0.5-1 cm so that the length will not belimited by optical loss (because it has lower optical loss thanP-doped). If we put in a QW structure that increases the nonlinearresponse by about 3 times from just the use of LEO, then the modulatorlength for such NIN structure can be ˜1.5 to 3 mm for achieving 1V. Ifwe then also reduce the modulator capacitor lateral width to 1 μm orless (to increase the frequency response), combined with the shorterlength for 1V, 100 GHz and 1V can then be reached concurrently.

Both these alternative NIN and NPIN (and the conventional PIN)structures are good for modulator devices that operate under reversebias, and are alternative exemplary embodiments for the modulators inthe present invention. Other alternative variations include reversebiased NP‘I’N, N‘P’IN, N‘P’‘I’N, NP‘I’N, P‘I’N, P‘I’N, P‘I’N, ‘P’‘I’N,or P‘I’‘N’ structures; or PNIP, PN‘I’P, P‘N’IP, P‘N’‘I’P, PN‘I’P or PIP,P‘I’P, ‘P’‘I’P, N‘I’N, ‘N’‘I’N structures, where quantum wells areplaced in the ‘P’, ‘N’, and ‘I’ layers (those layers in inverted commas‘X’).

Alternative Electro-Active Layer Structure B: P‘N’N Structure

The PIN (or NIN, NPIN) structures, while attractive in some ways, aresuffering from their relatively long device length of over 1 mm, whichwill become challenging when one tries to reach very high frequency suchas 100 GHz as in that case the electrodes' RF velocity matching with theoptical velocity will have to be good, which is potentially possible toengineer but will make manufacturing more complex and costly. It wouldbe desirable to reduce the modulator length to a much shorter length. Itis also desirable to achieve a lower voltage of <0.5V. It turns out thatit is possible to do so with use of the P‘N’N structure described below.

Another exemplary embodiment of the active-layer structure in themodulator structure of the present invention makes use of a novel P‘N’Nstructure with or without QWs. Again strongly confined waveguide is usedto reduce the voltage by for example 5 times.

The use of P‘N’N structure involves doped layers with or without QWs.Reverse bias is applied to the P‘N’N junction (with positive voltage tothe N side). The center ‘N’ layer is doped with or without QWs. Thedoping enables carrier band-filling and plasma effects to be used toincrease the phase shift. It enables BF+PL and also QCSE effects to beused beside the LEO effect. When properly designed, this enables thenonlinear EO response to be many times higher than that with the use ofLEO plus the typical QW QCSE. For EA modulation, this enables the EAresponse to be many times higher than that with the use of the typicalQW QCSE.

Basically as is known to those skilled in the art, there is a carrierdepletion layer at the P‘N’ junction (the carrier depletion is in adirection perpendicular to the P‘N’ junction creating a smallcarrier-free region “D” resulting in PD‘N’ in terms of carrier densitydistribution where D is depleted of carriers). Under reverse bias, thewidth of the depletion region (D) will increase. This means the carrierband-filing in the D region is modified, resulting in a change in therefractive index (at long wavelength of 1550 nm) due to BF effect. Ifquantum wells are placed in the D region, they will enhance therefractive index change due to BF. If quantum wells are used, there willbe a change in the optical absorption also (at energy above the quantumwells' lowest energy level) due to band filling as well. When operatingas EO modulator (i.e. based on the refractive index change), theoperating wavelength will be at an energy way below the quantum wells'lowest energy level so the optical absorption or optical absorptionmodulation will be minimal, and the main optical modulation effect willbe due to refractive index modulation.

Further optimizing the design of the QW structure and doping can pushthe electro-optic or electro-absorption response even higher, but aprice to pay for the optimized P‘N’N structure is higher optical loss,which restricts the length of the modulator to be shorter than 1.0 mm.The higher EO or EA response, however, more than made up for the shorterlength. A good design we have computed for the P‘N’N+QW structure for EOresponse gives 1 Volt at 0.2 mm length and 50 GHz bandwidth (atmodulator-capacitor width of w_(EC)=0.7 μm). The length is shorter thanthe 50 GHz RF wavelength, which is around 2 mm. As a result, it reducesthe RF-optical velocity matching requirement and makes it easier tofabricate and easier to reach 50 GHz. It also opens up various excitingpossibilities. For example, 0.25V is also achievable when a longerdevice length of less than 1.0 mm is used.

This P‘N’N modulator will be another exemplary embodiment of theactive-layer structure for the modulators of the present invention.Other alternative variations include reverse biased P‘P’N, structure(typically N doped is preferred for the middle layer for achieving loweroptical loss but P doped is also possible). Other variations include‘P’‘N’N or P‘P’‘N’ in which quantum wells are placed in both the ‘P’ and‘N’ layers.

Alternative Electro-Active Layer Structure C: NP‘N’N Structure

The short device length advantage of the P‘N’N structures can becombined with the lower optical loss and lower electrical contactresistance advantage of NPIN (or NIN) structures to give both lowoptical loss and high modulation, by utilizing a NP‘N’N structuredescribed below. The NPIN (or NIN) structure only requires N Ohmiccontacts as the two other layers contacting the metal contacts are bothN-doped. As no P Ohmic contact is needed, it will reduce the totalelectrical series resistance of the modulator structure substantially,leading to higher frequency response. The material layers with P-dopantwill also be thinner or fewer, which will also reduce the free-carrierinduced optical absorption loss and further reduce the electrical seriesresistance as well (P-doped layer typically has 10 times higher opticalabsorption and 10 times higher electrical resistance than N-doped layerwith the same carrier doping density).

Another exemplary embodiment of the active-layer structure in themodulator structure of the present invention makes use of a novel NP‘N’Nstructure with or without QWs. Again strongly confined waveguide is usedto reduce the voltage by for example 5 times.

The quantum wells, if used, are typically in the ‘N’ layer but can alsobe in any of the N or P layers, including the transition region betweenthe ‘N’ and any of the two P or N layer, or in plurality of theselayers. The active medium can be one or more of the layers (N, or P, or‘N’ layer), or the quantum-well structure, or other active-mediumstructure embedded in the electro-active layer, or a combination ofeffects from the quantum wells and one or more of the layers (N, or P,or ‘N’ layer).

The use of NP‘N’N structure involves doped layers with or without QWs.Reverse bias is normally applied to the P‘N’ junction but with bothsides being N contacts (with positive voltage on the N side of the P‘N’junction). Thus the NP junction is forward biased. The center ‘N’ layerhas doped layers with or without QWs. The doping enables carrierband-filling and plasma effects to be used to increase the phase shift.It enables BF+PL and also QCSE effects to be used beside the LEO effect.When properly designed, this enables the nonlinear EO response to bemany times higher than that with the use of LEO plus the typical QWQCSE. For EA modulation, this enables the EA response to be many timeshigher than that with the use of the typical QW QCSE.

Further optimizing the design of the QW structure and doping can pushthe electro-optic or electro-absorption response even higher, but aprice to pay for the optimized NP‘N’N structure is higher optical loss,which restricts the length of the modulator to be shorter than 1.0 mm.The higher EO or EA response, however, more than made up for the shorterlength. A good design we have computed for the NP‘N’N+QW structure forEO response gives 1 Volt at 0.2 nm length and 100 GHz bandwidth (atmodulator-capacitor lateral width of w_(EC)=0.7 μm). The length isshorter than the 100 GHz RF wavelength, which is around 1 mm. As aresult, it reduces the RF-optical velocity matching requirement andmakes it easier to fabricate and easier to reach 100 GHz. It also opensup various exciting possibilities. For example, 0.25V is also achievablewhen a longer device length of less than 1.0 mm is used. In addition, itcan achieve lower optical loss and lower electrical contact resistancethan the P‘N’N structure.

This NP‘N’N modulator will be another exemplary embodiment of theactive-layer structure for the modulators of the present invention. TheNIN is simpler but the NP‘N’N has better low-voltage and high-frequencyperformances and also almost as low an optical loss and as low theelectrical contact resistance as the NIN structure. Other alternativevariations include reverse biased NP‘P’N, structure (typically N dopedfor the ‘X’ layer is preferred for achieving lower optical loss but Pdoped is also possible).

Still other alternative variations include N‘P’‘N’N, N‘P’NN for which ifQWs are used, they will be in both the ‘P’ and ‘N’ layers. For N‘P’‘N’Ncase, under reverse bias, the depletion width between the ‘P’‘N’junction will widen and will sweep out carriers from the quantum wellsin both the ‘P’ side and the ‘N’ side, resulting in twice the refractiveindex change than if the quantum wells are only in the ‘N’ layer such asin a NP‘N’N structure.

Other alternative variations include reverse biased NP‘P’N; or PNIP,PN‘P’P, P‘N’PP, P‘N’‘P’P, or PN‘N’P structures; where quantum wells areplaced in the ‘P’, ‘N’, and ‘I’ layers (those layers in inverted commas‘X’).

Alternative Electro-Active Layer Structure D: Forward BiasedNN(+)P(+)PIN Structure

In the forward biased case, the low-optical-loss andlow-electrical-resistance advantages of NIN structure may be achieved,by utilizing a NN(+)P(+)PIN structure described below.

Another exemplary embodiment of the active-layer structure in themodulator structure of the present invention makes use of a novelNN(+)P(+)PIN structure. Again strongly confined waveguide is used toreduce the voltage by for example 5 times.

The active medium is typically in the I layer but can also be in any ofthe N or P layer, including the transition region between the I and anyof the N or P layer, in the other doped N(+) or P(+) or N or P layer, orin plurality of these layers. The active medium can be the layer itself,or a quantum-well structure, or other active-medium structure embeddedin the electro-active layer.

Forward bias is normally applied to the PIN junction (with positivevoltage on the P side of the PIN junction). In that case, the N(+)P(+)junction is formally under reverse biased, which normally would not havemuch current flow. However, as is known to those skilled in the art,when the N(+) and the P(+) layers are highly doped (typically at adoping density of higher than about 1×10¹⁸/cm³ and preferably higherthan 1×10¹⁹/cm³ for both the N and P material, the carrier can actuallytunnel through under the reverse bias, resulting in current flow throughthe N(+)P(+) junction, into the PIN junction area that is forwardbiased. In that case, the N(+)P(+) junction is normally referred to as acarrier tunneling junction (or simply as tunnel junction). Such tunneljunction can be very thin with the N(+) and P(+) layer only tens ofnanometers in thickness each. The net result is the changing of the POhmic contact at layer P to N Ohmic contact at layer N of the NN(+)side. As noted above, N Ohmic contact generally has a much lower (10times lower) contact resistance than P Ohmic contact. The use of suchpair of N(+)P(+) tunnel junction layers thus enables one to have N Ohmiccontacts on both sides of the device. This structure also works if onlyelectric field is wanted at the active medium (i.e. with PIN junctionunder reverse bias).

This NN(+)P(+)PIN structure will be another exemplary embodiment of theactive-layer structure for the devices of the present invention. Otheralternative variations include NN(+)P(+)IN, N(+)P(+)PIN, N(+)P(+)INstructures and the likes or with some doping in the active-medium layertypically in the intrinsic layer resulting in NN(+)P(+)P‘N’N,NN(+)P(+)‘P’‘N’N, NN(+)P(+)‘N’N, N(+)P(+)P‘N’N, N(+)P(+)‘P’‘N’N,N(+)P(+)‘N’N, NN(+)P(+)P‘P’N, NN(+)P(+)‘P’N, N(+)P(+)P‘P’N, orN(+)P(+)‘P’N structures; or PIN, PP(+)N(+)NIP, PP(+)N(+)N‘P’P,PP(+)N(+)‘N’‘P’P, PP(+)N(+)‘P’P, P(+)N(+)N‘P’P, P(+)N(+)‘N’‘P’P,P(+)N(+)‘P’P, PP(+)N(+)N‘N’P, PP(+)N(+)‘N’P. P(+)N(+)N‘N’P, orP(+)N(+)‘N’P structures; where quantum wells are placed in the ‘P’, ‘N’,and ‘I’ layers (those layers in inverted commas ‘X’). They are good fordevices that operate under forward bias for the PIN (or P(+)IN or PIN(+)or P(+)IN(+)) junction. They are also good for devices that operateunder reverse bias for the PIN (or P(+)IN or PIN(+) or P(+)IN(+))junction, and are alternative exemplary embodiments for the activephotonic devices in the present invention.

When the context is clear below, we will drop the inverted commas in ‘N’or ‘P’ designations in the electro-active layer structures above. Theabove examples are for the purpose of illustrations and not limitations.For example, the various doped structures may also be joint one on topof another forming a cascaded structure. Those skilled in the art willknow other obvious variations that are variations of the above examplesof the various doped structures with or without the use of quantumwells.

Slow-Wave Electrode Structure for Velocity+Impedance Matching

In certain applications such as traveling-wave modulator, atravelling-wave RF transmission line electrode structure should befabricated along the device waveguide. Such traveling-wave RFtransmission line electrode structure is often needed in order toachieve high-frequency response of 10-100 Gb/s or higher for themodulator. Below describe such a travelling wave electrode and theiroptimization to match the velocity of propagation of the optical beamand the RF wave. In such travelling wave electrode, it is oftenadvantages to engineer the electrode impedance to be around the standardimpedance of 50Ω or some other preferred value depending on theapplication.

As the RF dielectric constant in III-V semiconductors is close to theirdielectric constant at optical frequency, and the RF wave in the case ofsemiconductor modulators tends to have electric field fringing to thesurrounding materials with lower dielectric constant, the RF wave tendsto propagate at a faster velocity than the optical wave. This can bemanaged by using an adjustable slow-wave capacitively-loaded travelingwave (CL-TWE) RF transmission-line structure as shown in FIG. 5a . Theprice to pay is a longer length, trading off mainly optical loss (notmuch of RF loss) but it allows impedance matching.

It turns out that the slow-wave structure also enables freedom toengineer concurrent impedance matching to 50Ω as there is freedom tochoose its filling factor “F” that will change its effectiveinductance-length product L and capacitance per unit length C. Itusually ended up with slightly larger voltage-length product than if theimpedance is allowed to be lower than 50Ω, resulting in longer lengthfor the same modulation voltage. Most of the structures above haveplenty of rooms to absorb the longer length. Hence, velocity matchingand 50Ω can be engineered. However, velocity matching is less importantwhen the modulator is shorter than 0.2 mm as the RF wavelength at 100GHz is about 1 mm.

Exemplary Device for the Modulators of the Present Invention

The exemplary device below illustrates a particular exemplary embodimentof the EO modulator of the present invention, including thetravelling-wave electrode, the Mach Zehnder Interferometer, and pushpull geometry. The general scheme for the modulator is illustrated inFIG. 4. In one particular exemplary embodiment and realization of suchmodulator, the said modulator can be fabricated on a “silicon photonics”platform for operation at the fiber-optic communication wavelength ofaround 1550 nm. The modulator is fabricated on a silicon-on-insulator(SOI) wafer for which a thin layer (about 300 nm thick) of silicon ispre-bonded onto a thermal silicon oxide layer with a thickness of 1-2micrometers grown on top of a bottom silicon substrate. The 300 nm thicktop-layer silicon on SOI wafer serves as an on-chip optical waveguide,which can be fabricated into the channel waveguide shown in FIG. 4b ,for which the optical beam is propagated in the channel waveguide asshown in FIG. 4b . In this particular embodiment, the channel waveguidecore is made of high-refractive-index silicon (with a refractive indexn_(core)˜3.6). The waveguide core is surrounded by silicon dioxide orair with much lower refractive index of n_(clad)˜1 to 1.5, that act aswaveguide cladding. The use of silicon as waveguide enables electronicintegrated circuits to be fabricated on the same chip. Such waveguidesare well known to those skilled in the art and are often referred to as“silicon photonics” platform. If active photonic devices such asmodulators are then fabricated, the entire platform is some timereferred to as Electronic-Photonic Integrated Circuits (EPICs).

For the modulator shown in FIG. 5a , a pair of travelling-wave RFtransmission line electrodes is used to propagate the RF electricalsignal along the EO modulator. These electrodes are designed so that theRF wave propagates at a velocity close to the optical wave velocity inthe modulator's optical waveguide. This is referred to as velocitymatched transmission line. As is known to those skilled in the art,velocity matching is desirable when the modulator length is longcomparing to the RF wavelength so that maximum modulation can beachieved in the optical beam.

In this example for the case of an EO modulator geometry, an opticalbeam is further split into two propagating arm forming a Mach ZehnderOptical Interferometer (MZI). Each arm has an EO phase modulator. As isknown to those skilled in the art, it is typically arranged so that theRF wave will cause positive phase shift in one of the arm and negativephase shift in the other arm, resulting in what is known as “push-pull”configuration for the modulator. As is known to those skilled in theart, a number of configurations can be used beside the push pullconfiguration. Thus such RF electrode and MZI geometry are shown onlyfor the purpose of illustration and not limitation.

An embodiment of the present invention is focused on the realization ofan EO phase modulator that can be used as shown in a MZI geometry or asan isolated phase modulator for various applications as is well known tothose skilled in the art. The RF electrodes illustrate an exemplaryembodiment in a particular application of the modulators of the presentinvention.

For this particular exemplary embodiment, the RF electrodes are a pairof capacitively-loaded traveling wave electrodes (CL-TWE). Each arm ofthe CL-TWE is electrically in contact with electrode traveling along anoptical waveguide based EO optical phase modulator. The EO optical phasemodulator is reverse biased by a DC applied voltage. Thus there are twophase modulators, one power by each arm. The two optical waveguidingarms then form a Mach Zehnder optical Interferometer (MZI) geometry foran input optical beam. The input optical beam is split with half thepower going to each arm. At the output, the phase modulated beams fromthe two phase modulator outputs are then combined again giving theoptical output for the MZI. As is well known to those skilled on theart, the MZI enables the phase modulation to be converted to opticalintensity modulation at the beam combined optical output of the MZI. Inthe case of a push-pull geometry, as is known to those skilled in thearm, the two arms will receive opposite optical phase shift, enablingdouble the intensity modulation under the same applied voltage to thetwo phase modulators, one on each arm of the MZI.

In order to realize a push-pull geometry, in terms of powering the twoarms of the Mach Zehnder Interferometer (MZI), the illustrated schemefollows that of the PIN modulator. As shown in FIG. 5a , RF wave islaunched into the CL-TWE from one end of the CL-TWE. The segmentedperiodic pairs of T-shaped electrodes bring the voltage from the mainslotted electrodes to the modulator device. Each pair is separated by aperiodic length l_(p)=l_(m)+l_(g) and each of the T-shape segment has alength l_(m). The fill factor F is defined as F=l_(m)/l_(p) (see FIG. 5a), where “A/B” denotes division of A “over” B. Typically l_(p)=50-100μm. Along the gap l_(g)=l_(p)−l_(m), the modulator section has noapplied voltage.

In this particular exemplary embodiment, the bottom parts of the twophase modulators are connected. This is shown in FIG. 5b showing thecross-section f-f′ in FIG. 5a . Thus, any voltage Vs applied is split+Vs/2, and −Vs/2 on the two arms forming a push-pull pair of phasemodulators. Unlike the conventional push-pull modulators, there is nonet voltage reduction by half as the line voltage Vs is twice thevoltage applied to each modulator Vs/2. However the junctioncapacitances (C_(j) in FIG. 5a ) of the two phase modulators areconnected in series, reducing the total capacitance or increasing thefrequency, so there is a gain over the conventional push-pull EOmodulator in broader frequency response (at the expense of highervoltage).

The equivalent lumped-element circuit of such a CL-TWE powering the twophase modulators is shown in FIG. 6a , in which the dotted line labeledas “Transmission Line” represents the basic unloaded transmission linemodel for the CL-TWE. The solid line part labeled as “Device Load” takesinto account RF propagation loss of all the modulator structuralelements including: (1) Metal Ohmic Contact Resistance and CapacitanceZ; (2) Resistance and Capacitance Z_(d) of the Doped Waveguide Layersincluding those in the TCO materials (if TCO is used); (3) Modulatoractive-region Capacitance C_(j); (4) Both Transverse and LongitudinalCurrent losses in the Semiconductor Structure are included. The modelgives us the frequency bandwidth limited by the RF losses (for longmodulator) or by the RC time constant (for short modulator), and thetransmission line impedance. It also gives us the CL-TWE filling factorF needed to achieve velocity matching.

FIG. 6b shows the top view of the CL-TWE lines for the traveling waveelectrodes. The voltage V_(b) in FIG. 6b connecting between the bottomcommon contact and the top of one of the modulators givingV_(b1)=V_(b2)=V_(b) (when Vs=0) is used to reverse bias the modulators.DC offset in Vs gives differential biasing that can be used to tune theoperating point of the Mach Zehnder optical interferometer by tuning thedifferential phase shift imposed by the two phase modulators along itstwo arms.

As is known to those skilled in the art, there are many other electrodestructures that can be used. The above illustrates one embodiment of atraveling wave structure that can slow down the propagation velocity ofthe RF wave so as to achieve better velocity matching with the opticalbeam in the modulator (it is some time referred to as slow-wave RFtraveling-wave structure). Such velocity matching will help to achievehigher frequency response as is known to those skilled in the art. Theabove exemplary embodiment on the traveling-wave electrodes is shown forthe purpose of illustrations and not limitation.

In terms of the optical beam, it enters the silicon waveguide from anoptical fiber. There are many ways to couple optical beam from anoptical fiber to silicon waveguide as is known to those skilled in theart such as via an integrated mode size transformer on silicon calledSuper-High-Numerical Aperture Graded Refractive Index (SuperGRIN) lens,which will efficiently couple the beam power from the optical fiber tothe 300 nm thick silicon waveguide on a SOI substrate (SOI waveguide).Alternative fiber to silicon waveguide couplers such as tapering downwaveguide or surface grating can also be used as is well known to thoseskilled in the art.

The beam from the SOI waveguide is then coupled into the thin-filmmodulator structure as shown by FIG. 7 a/b using two tapers (one on theactive material layer (e.g. InP/III-V), one on the primarily passivewaveguide layer (e.g. Si)—see the top view). Due to the thin-thicknessof the InP/III-V modulator's active layer structure (only 300-400 nmthick) for operation at 1550 nm wavelength range, this taper section canbe very short (like 5-30 □m depending on the thickness) and over 95% ofthe energy can be transferred into the thin film modulator device. Theoutput back into the optical fiber goes through a reverse process viatwo output tapers (one on the active material layer (e.g. InP/III-V),one on the primarily passive waveguide layer (e.g. Si)—see the top view)to bring the optical beam to the primarily passive waveguide layer (e.g.Si) and then to an output fiber-coupling lens.

The general structure for the NIN and NPNN modulators are also similar.What differentiate them are the detailed layer structures. There are twoversions of the general structure, one uses side-conduction layer (SCL),another Ohmic Transparent Conductor (OmTC).

SCL Case.

In a particular exemplary embodiment for application to 1550 nmwavelength range, the cross-section for the SCL case is shown in FIG. 5a. At the bottom, the modulator has a highly doped lower layer that isabout 100 nm thick for this particular exemplary embodiment. This layeris used to conduct the voltage side way and is called “bottom sideconduction layer” (BSCL). As noted above, in the push-pull modulatorcircuit scheme for the particular exemplary embodiment, this layer isconnected to the phase modulator at the other arm of the MZI.

Above this BSCL is an active EC layer structure that will be differentfor NIN and NPNN. It is then followed by a 100 nm thick top layer. Thistop layer is largely N-doped and is used to conduct voltage side way andis called the “top side conduction layer or TSCL”. On top of this at themiddle is deposited with 300 nm-thick low-refractive-index SiO₂. Bothsides of TSCL are deposited with metal. The center SiO₂ layer preventsoptical energy in the waveguide from touching the optically lossy metal.

The center active EC layer has to be narrow in width to make the devicecapacitance small as its width w_(EC) will define themodulator-capacitor width w_(CAP)=w_(EC). To enable fabrication, one wayis to use a material for this active EC layer that can be chemicallyselectively etched sideway without etching the material that formed theBSCL or the TSCL layers. Other fabrication schemes may be used as longas the small width for w_(EC) is achieved (e.g. by etching the requiredwidth for w_(EC), then depositing the material required for forming theTSCL layer). In order to achieve high frequency response, an exemplarymodulator structure requires w_(EC) to be around 2 μm=2000 nm or asnarrow as 0.7 μm=700 nm. While it may seem to be small (making itchallenging to fabricate), it is still larger than the typical thicknessof this thin-film modulator structure with a thickness of 300-400 nm.Thus, the width we still has a low aspect ratios (<1:3) with the othernearby structural parameters. Low aspect ratios make it not toodifficult to fabricate. It can be done by careful control of theetching. Both sides of the metal contact will be around 2 μm=2000 nm inorder to have a large enough Ohmic contact area with metal so as to havesmall enough metal contact resistance.

OmTC Case.

In a particular exemplary embodiment for application to 1550 nmwavelength range, the cross-section for the OmTC case is shown in FIG.8b . In the case of a OmTC based structure, there will be no TSCL. Thus,in this particular exemplary embodiment, the top layer thickness can bereduced by about 60 nm, leaving 40 nm for Ohmic contact with TC(Transparent Conductor) material in general or TCO (TransparentConducting Oxide) material in particular. This 60 nm thickness can beadded to the bottom BSCL making it 160 nm thick resulting in lowerresistance and more robust structure. The top layer contacting the TC isN-doped for achieving good Ohmic contact. In this particular exemplaryembodiment, the TC is In₂O₃. As the TC generally has low refractiveindex (e.g. n˜1.7), the optical field energy will decay rapidly in theTC and will not touch the metal much. As TC can be deposited after thedry etching to form the width W_(EC) needed for the electro-active (EC)layer, no side etching of the center electro-active layer (ECL) isneeded. We just need to etch vertically for the require thickness andthen planarize with polymer, then deposit with TC. Hence, this OmTC casewill enable a structure that is more robust, with lower resistance, andeasier to fabricate than the SCL case.

Detailed Structures for NIN and NPNN Modulators

NIN Structures.

In a particular exemplary embodiment for application to 1550 nmwavelength range, the detail structures for a NIN modulators are shownin FIG. 9. In this particular exemplary embodiment, the structure has ahighly-doped bottom InP N-layer about 120 nm thick (this becomes theBSCL) followed by a 195 nm thick intrinsic layer made of AlGaInAs QWstructure (with λ_(QW)˜1350 nm), and then by another highly-doped topInP N-layer about 125 nm thick for side contact with metal. This gives atotal thickness of 440 nm. For this structure, the active EO regionwidth w_(EC) or the modulator-capacitor width w_(CAP) defined by it(w_(CAP)=w_(EC)) is chosen to be 2,000 nm (2 μm). This width also givesthe lateral optical mode confinement as shown by FIG. 10b . In FIG. 10a, we show the mode of a conventional modulator in comparison. The NINmode in FIG. 10b is 4 times smaller in the vertical direction, givingaround 4 times larger mode-medium overlapping factor.

The computed modulator length for Vπ=1V and F=1 is L_(MOD)=2 mm.

Table 1 show the material layer structure for this NIN structure basedmodulators with side metal contact for its top contact, spelling out thethicknesses and bandgap energies of the compound semiconductor materialin each layer with the various doping density and strain (with InP asthe substrate).

TABLE 1 Layer Layer Number Thickness NIN CASE Metal Doping 1 120 nm InP(Bottom Layer-just n = 1 × above the substrate) 10{circumflex over( )}19 2  5 nm AlGaInAs 1.3 um I 3 5 nm barrier AlGaInAs/1.1 um/−0.8% Itensile strained 4 2 × 7 nm AlGaInAs/1.1 um/−0.8% I barrier insidetensile strained 5 3 × 6.5 nm Well AlGaInAs/1.55 um/0.9% I (PL = 1350nm) compressive strained 6 5 nm barrier AlGaInAs/1.1 um/−0.8% I tensilestrained 7  60 nm AlGaInAs 1.3 um I 8 125 nm InP (Top Layer) n = 1 ×10{circumflex over ( )}19 Total 440 nm

NPNN Structure.

In a particular exemplary embodiment for application to 1550 nmwavelength range, as shown in FIG. 10, a NPNN structure has ahighly-doped bottom InP N-layer about 160 nm thick, followed by a 115 nmthick active Electro-Active layer (ECL) made of AlGaInAs QW structure,then by a thin layer of InP P-doped layer (around 25 nm), then by ahighly-doped top InP N-layer about 80 nm thick for top contact toOm-TCO. This gives a total thickness of 380 nm. The Electro-Activeregion width w_(EC) is chosen to be 2,000 nm (2 μm).

TABLE 2 Layer Layer Number Thickness NPNN TCO CASE Doping 1 160 nm  InP(Bottom layer-just n = 1 × above the substrate) 10{circumflex over( )}19 2 10 nm AlGaInAs 1.3 um n = 4 × 10{circumflex over ( )}17 3 4 nmbarrier AlGaInAs/1.1 um/−0.8% n = 4 × tensile strained 10{circumflexover ( )}17 4 2 × 7 nm AlGaInAs/1.1 um/−0.8% n = 4 × barrier insidetensile strained 10{circumflex over ( )}17 5 3 × 6.5 nm WellAlGaInAs/1.55 um/0.9% n = 4 × (PL = 1350 nm) compressive strained10{circumflex over ( )}17 6 4 nm barrier AlGaInAs/1.1 um/−0.8% n = 4 ×tensile strained 10{circumflex over ( )}17 7 63 nm AlGaInAs 1.3 um n = 4× 10{circumflex over ( )}17 8 25 nm InP p = 1 × 10{circumflex over( )}18 9 80 nm InP (Top layer) n= 1 × 10{circumflex over ( )}19 Total380 nm 

As shown in FIG. 10, comparing to NIN case, the main difference for thisparticular exemplary embodiment of a NPNN case is that the active EClayer has QWs that are also doped with N-type carriers and it is next toa bottom p-doped layer. Under a reverse bias, the depletion widthbetween the p-doped layer and the N-doped QW region is enlarged withvoltage. The voltage drop across this depletion region then imposes aconstant electric field on the QW. The increase in phase shift isbecause more QWs are applied with the electric field when the voltageincreases. The doped carriers also cause phase shift due to band-filling(BF) and Plasma (PL) effects. When optimized, the BF+PL+LEO+QCSE canlead to 3-5 times larger phase shift than LEO+QCSE. The thinnerstructure of 380 nm comparing to NIN of 440 nm also reduces themodulator length by another 20%. The computed modulator length for Vπ=1Vis L_(MOD)=0.4 mm.

Table 2 shows the material layer structure for the NPNN structure basedmodulators with Om-TCO contact for its top contact, spelling out thethicknesses and bandgap energies of the compound semiconductor materialin each layer with the various doping density and strain (with InP asthe substrate).

The Role of F for Velocity & Impedance Matching

In a particular exemplary embodiment for application to 1550 nmwavelength range, the lumped-element model shown in FIG. 6a is used toestimate the modulator device performances. We use a Table to summarizethe different cases. We will illustrate some plots using a NIN case withSCL, and NPNN case with OmTC.

NIN CASE with SCL.

In a particular exemplary embodiment, the modulation BW for the NIN casewith 2-μm-wide active region so w_(CAP)=2 μm is shown in FIG. 11.

A calculation shows that for the NIN structure, in order to reach 1Vmodulation voltage, the modulator length shall be 2 mm. FIG. 11a showsthe net frequency response limited by RF loss (in dB/mm). Taking the3.24 dB/mm loss line in FIG. 11a , one finds a frequency responsebandwidth of 60 GHz for the NIN structure with side metal conduction.These are for w_(CAP)=2 μm. FIG. 11b shows the impedance for different Fvalues as a function of the frequency. FIG. 11c shows the propagatingrefractive index of the RF wave along the RF transmission line fordifferent F values as a function of the frequency illustrating F˜0 makesthe RF refractive index close to 3.5, which is nearly matching thepropagating refractive index of the optical beam in the modulator(typically around n˜3 to 3.5), which is required for velocity matching.

NPNN Case with OmTC.

In a particular exemplary embodiment, the modulation bandwidth for theNPNN case with 2-μm-wide active region so w_(CAP)=2 μm (this widthdetermines the Modulator Capacitance C_(j)) is shown in FIG. 12.

A calculation shows that for the NPNN structure, in order to reach 1Vmodulation voltage, the modulator length shall be 0.4 mm. FIG. 12a showsthe net frequency response limited by RF loss (in dB/mm). Taking the34.2 dB/mm loss line in FIG. 12a , one finds a frequency responsebandwidth of 100 GHz for the NPNN structure with Ohmic TCO as topcontact. These are for active-region width of 2 μm. FIG. 12b shows theimpedance for different F values as a function of the frequency. FIG.12c shows the propagating refractive index of the RF wave along the RFtransmission line for different F values as a function of the frequencyillustrating F˜0.4 makes the RF refractive index close to 3.5, which isnearly matching the propagating refractive index of the optical beam inthe modulator (typically around n˜3 to 3.5), which is required forvelocity matching.

More Detailed Descriptions of the Various Embodiments of the PresentInvention

A schematics showing the general geometry of the Active Photonic Devicesof the present invention is shown in FIG. 13. FIGS. 13a and 13b show aActive Photonic Device 20000 with a modulating section or referred tobelow as “Active-Layer Structure ALS 22500” section, coupled to an inputconnecting waveguide core ICWCo 22200. FIG. 13b is a semi-transparentillustration of FIG. 13a . The input connecting waveguide core ICWCo22200 is fabricated on an input connecting-waveguide bottom claddingmaterial ICWBCd 22200B disposed on a substrate SUB 21100.

In one exemplary embodiment shown in FIG. 14, illustrating an exemplaryembodiment of the cross section at location A-A′ of FIG. 13, to the topof the input connecting waveguide core is further deposed with inputconnecting waveguide top cladding material ICWTCd 22200T. Bottom refersto direction closer to the substrate and top refers to direction awayfrom the substrate. In FIG. 14, to the left of the input connectingwaveguide core is also deposed with input connecting waveguide leftcladding material ICWLCd 22200L and to the right of the waveguide coreis disposed with input connecting waveguide right cladding materialICWRCd 22200R.

Right or left is defined by taking the direction of beam propagation asthe front direction and right or left means relative to this frontdirection. The division of the cladding into four different materialregions is for the purpose of discussion and not limitation as these canbe all the same materials or there can be more than 4 regions formingplurality of different regions as long as these regions act as waveguidecladding materials with effective refractive indices smaller than theeffective refractive index of the waveguide core ICWCo 22200 so that thelight beam power is confined mainly in the region of the waveguide coreICWCo 22200, as is well known to those skilled in the art. The claddingmaterial is a general designation that can include “air” or “vacuum” orany transparent dielectric material as the material.

Input Connecting Waveguide Region

The input connecting waveguide core ICWCo 22200 is made up of a materialor mixture of materials with an averaged material refractive indexn_(ICWCo) 22200 n, has a thickness d_(ICWCo) 22200 d, and widthW_(ICWCo) 22200 w. Let the refractive index of the bottom inputconnecting-waveguide bottom cladding material be n_(ICWBCd) 22200Bn. Letthe refractive index of the top cladding material ICWTCd 22200T ben_(ICWTCd) 22200Tn, the refractive index of the left cladding materialICWLCd 22200L be n_(ICWLCd) 22200Ln, and the refractive index of theright cladding material ICWRCd 22200R be n_(ICWRCd) 22200Rn. Thewaveguide core 22200 and the claddings 22200T, 22200B, 22200R, 22200L,together forms input connecting waveguide ICWG 22200WG.

The vertical confinement of the optical beam is due to therefractive-index difference between the top and bottom waveguidecladdings and the waveguide core and the claddings generally have lowerrefractive indices than that of the waveguide core so thatn_(ICWTCd)<n_(ICWCo) and n_(ICWBCd)<n_(ICWCo). The horizontalconfinement of the optical beam is due to the refractive-indexdifference between the left and right waveguide claddings and thewaveguide core and the claddings generally have lower refractive indicesthan that of the waveguide core so that n_(ICWRCd)<n_(ICWCo) andn_(ICWLCd)<n_(ICWCo). The vertical direction is the directionperpendicular to the substrate plane and the horizontal direction is thedirection parallel to the substrate plane.

The above illustration of an exemplary embodiment of input connectingwaveguide ICWG 22200WG, showing the waveguide cladding can be dividedinto different material regions (in the above case with four mainmaterial regions), is for the purpose of illustration and notlimitation. As is known to those skilled in the art, the waveguidecladding can be made up of one single material or plurality of materialregions, as long as the refractive indices of most of the claddingmaterial regions is lower than the refractive index n_(ICWCo) of thewaveguide core. This is also generally applicable to the other waveguidecladding situations below for other optical waveguides described in thepresent invention.

Definition of Refractive Index Contrast and Cladding Refractive IndexAveraging

An important quantity in terms of waveguide mode confinement is therefractive index contrast between the averaged refractive index of thewaveguide core and its immediate surrounding cladding materials calledthe refractive-index difference n_(Rd) defined by n_(Rd) ²=□n_(Co)²−n_(Cd) ²), where n_(Co) is the refractive index of the waveguide core(e.g. n_(Co)=n_(ICWCo)) and n_(Cd) is the refractive index of thewaveguide cladding (e.g. n_(Cd)=n_(ICWBCd) or n_(ICWTCd) or n_(ICWRCd)or n_(ICWLCd)) or an averaged of them thereof given by n_(aICWCd) 22200aCdn.

The refractive-index averaging is more accurately done as averaged ofits square values which are their dielectric constant ∈=n². This isbecause dielectric constants which describe the dipole strengths addlinearly with each other as is known to those skilled in the art. Thusn_(aICWCd) ² for example can be computed by weighting the refractiveindex square in each of the different cladding regions by the fractionof beam energy in each of the cladding regions. Hence:

n _(aICWCd) ²=(n _(ICWBCd) ² ×A _(ICWBCd) +n _(ICWTCd) ² ×A _(ICWTCd) +n_(ICWRCd) ² A _(ICWRCd) +n _(ICWLCd) ² A _(ICWLCd))/(A _(ICWBCd) +A_(ICWTCd) +A _(ICWRCd) +A _(ICWLCd)),  (13)

where in Eq. (13), A_(ICWBCd) is some effective cross-sectionalweighting for the optical power in the bottom cladding material (e.g.given by the percentage of the total beam power), A_(ICWTCd) is someeffective cross-sectional weighting for the optical power in the topcladding material, A_(ICWRCd) is some effective cross-sectionalweighting for the optical power in the right cladding material,A_(ICWLCd) is some effective cross-sectional weighting for the opticalpower in the left cladding material. A_(ICWBCd), A_(ICWTCd), A_(ICWRCd),and A_(ICWLCd) are called the effective beam power distribution areas inthe respective regions of the waveguide cladding materials. Each ofthese cross-sectional weighting has a value proportional to thefractional optical power (beam power integrated over the beamcross-sectional area of that region) in that region of the material forthe guided optical beam or is given by the integration over the beamenergy density (energy per unit volume) over the volume of that regionof the material assuming the volume is taken over a short propagationlength. These are some definitions of the effective cross-sectionalweighting labeled with prescript “A”. Many other equivalent butapproximate definitions of the effective cross-sectional weighting “A”can be used. Note n_(aICWCd) ²˜(n_(ICWBCd) ²+n_(ICWTCd) ²+n_(ICWRCd)²+n_(ICWLCd) ²)/4, if these weightings are about equal.

Likewise the waveguide core can also generally be made up of one orplurality of materials, and n_(ICWCo)=n_(aICWCo) can also be an averagedrefractive index of the “m” number of materials with slightly differentrefractive indices n_(ICWCo1), n_(ICWCo2), n_(ICWCo3) . . . n_(ICWCom),that made up the waveguide core materials where

n _(aICWCo) ²=(n _(ICWCo1) ² ×A _(ICWCo1) +n _(ICWCo2) ² ×A _(ICWCo2) +n_(ICWCo3) ² A _(ICWCo3) + . . . +n _(ICWCom) ² A _(ICWCom))/(A _(ICWCo1)+A _(ICWCo2) +A _(ICWCo3)+ . . . +_(ICWCom)),  (14)

In Eq. (14), each of the A_(ICWCo1), . . . , A_(ICWCom) is someeffective cross-sectional weighting A_(ICWCoj) for the optical power incore material with refractive index n_(ICWCoj), where j is one of 1, . .. , m. A_(ICWCo1)+A_(ICWCo2)+A_(ICWCo3)+ . . . +A_(ICWCom) are calledthe effective beam power distribution areas in the respective regions ofthe waveguide core materials.

Input Optical Beam

As shown in FIG. 13, an input optical beam IBM 22140 is launched intoinput connecting waveguide core ICWCo 22200. As the cladding regions ofinput connecting waveguide ICWG 22200WG or in fact of any waveguide aretypically ill-defined (the beam power can go into various depths anddirections into the cladding) as is known to those skilled in the art,an input optical beam IBM 22140 launched into waveguide ICWG 22200WG istaken to mean it is launched with its power centered essentially at thewaveguide core ICWCo 22200, so the reference to the beam being inwaveguide 22200WG or waveguide core 22200 will generally be usedinterchangeably below. As is known to those skilled in the art, theinput optical beam IBM 22140 will propagate in waveguide 22200 with a“propagating refractive index” n_(IBM) 22140 n. This propagatingrefractive index n_(IBM) 22140 n generally has a value smaller than thematerial refractive index n_(ICWCo) 22200 n of the waveguide core ICWCo22200 so that n_(IBM)<n_(ICWCo) as is known to those skilled in the art.Also, the propagating refractive index n_(IBM) 22140 n generally has avalue larger than the material refractive index n_(ICWQCd) 22200Qn ofthe waveguide cladding ICWQCd 22200Q where Q=T,B,L, or R, so that formost of the Q, n_(IBM)>n_(ICWQCd) to enable waveguiding, as is known tothose skilled in the art. The optical beam has an optical power given byP_(IBM) 22140P, electric field polarization given by E_(IBM) 22140E, andbeam effective area given by A_(IBM) 22140A.

In this invention, the propagating optical beam is generally assumed tohave a spread of optical wavelength centered at an operating wavelengthλ_(IBM) 22140L. For illustration and not limitation, the optical beammay be in the form of a train of optical pulses to transmit digitalinformation. The optical beam may also be made up of light wave of oneor plurality of (N) different frequency channels (λ_(IBM1), λ_(IBM2),λ_(IBM3), . . . , λ_(IBMN)) where N is an integer. When the optical beamis made up of plurality of frequency channels, the optical transmissionsystem or device is generally known as a wavelength divisionmultiplexing (WDM) optical system or device. Generally, the optical beamis made up of beam of light with a spectral width around the centeroperating wavelength λ_(IBM).

Input Beam Coupler Structure (IBCS) Region

FIG. 15a show the input beam coupler structure (IBCS) comprises at leasta tapering waveguide section (preferably tapering from wide to narrowbut can also maintain the same width or taper from narrow to wide aswill be elaborated below) connected to the input waveguide. Optionally,the active layer structure ALS on top of the input tapering waveguidesection can also be tapering in the form of an up taper (preferablytapering from narrow to wide in the direction toward the active layerstructure ALS, but can also maintain the same width or taper from wideto narrow). See for example FIG. 15 b.

Specifically, the input optical beam IBM 22140 enters from inputconnecting waveguide core ICWCo 22200 into an input connecting-waveguidetaper section with an input tapering waveguide core ITWCo 22300parameterized by a location z1 (FIG. 15a ), ITWCo-z1 22300 z 1, in whichthe width of a tapering waveguide core w_(ITWCo-z1) 22300 w-z 1 atdistance z1, measured from the beginning point of the taper, is changedfrom its input value at z1=0 w_(ITWCo-z1=0) 22300 w-z 1=0 ofw_(ITWCo-z1=0)=V_(ITWCo) 22200 to another width (that can be the samewidth) at z1>0 w_(ITWCo-z1>0) 22300 w-z 1>0. The thickness of thetapering waveguide core is d_(ITWCo-z1) 22300 d-z 1 and its refractiveindex is n_(ITWCo-z1) 22300 n-z 1. Typically, though not always,d_(ITWCo-z1) 22300 d-z 1 and n_(ITWCo-z1) 22300 n-z 1 are constant valuein z1 so we can drop the z1 designation with d_(ITWCo-z1)=d_(ITWCo)22300 d and n _(ITWCo-z1)=n_(ITWCo) 22300 n. In a preferred embodiment,d_(ITWCo-z1) is about the same value as d_(ICWCo). The tapering may besuch that w_(ITWCo-z1) is a linear function of z1 or quadratic functionof z1 (i.e. depending on z1²), but can also be of any curvilinearfunction of z1. Let g_(ITWCo) 22300 g denotes the total length of thistapering waveguide.

The end of the taper at z1=g_(ITWCo) 22300 g at which the width of thewaveguide core is w_(ITWCo-g) 22300 w-g is connected to an inputsupporting structure ISTR 21200. While illustrated as a line that iscontinuation of the connecting waveguide material with a narrow widthand air or other low refractive index materials surrounding its side,the supporting structure can be random dots or any shape of small amountof any materials that have an “effective refractive index” or small“averaged refractive index” (e.g. as defined by Eq. (13)) within thelayer extended in the horizontal direction, given by an effective layeraveraged refractive index n_(laISTR) 21200 nla. In the case it acts asthe bottom waveguide cladding, n_(laISTR) has a value lower than therefractive index of the waveguide core n_(WCo) 22600Con in theelectro-active waveguiding core structure EWCoS 22600 defined below. Theinput supporting structure ISTR 21200 may continue to guide wave or justacts as a supporting structure, depending on application scenarios.

In an exemplary embodiment, the input supporting structure ISTR 21200 isa narrow line. In that particular case, we can describe it as having awidth w_(ISTR) 21200 w, thickness d_(ISTR) 21200 d, and length g_(ISTR)21200 g. The length g_(STR) 21200 g may be zero. In that case, inputsupporting structure ISTR 21200 does not exist (the thin ALS film canstill be supported in some way such as by its corners or sides, but notdirectly below). In a preferred embodiment, d_(ISTR) is about the samevalue as d_(ICWCo).

Along the taper in region outside the ALS region, the verticalconfinement of the optical beam is due to the refractive-indexdifference between the waveguide core and top and bottom taperingwaveguide claddings at the location z1 defined above: ITWTCd-z122300T-z1 (refractive index n_(ITWTCd-z1) 22300Tn-z1) and ITWBCd-z122300B-z1 (refractive index n_(ITWTCd-z1) 22300Bn-z1) and the waveguidecore and the claddings have lower refractive indices than that of thewaveguide core so that the refractive index n_(ITWTCd-z1)<n_(ITWCo-z1)and n_(ITWBCd-z1)<n_(ITWCo-z1). The horizontal confinement of theoptical beam is due to the refractive-index difference between the leftand right waveguide claddings at z1: ITWLCd-z1 22300L-z1 (refractiveindex n_(ITWLCd-z1) 22300Ln-z1) and ITWRCd-z1 22300R-z1 (refractiveindex n_(ITWRCd-z1) 22300Rn-z1), and the waveguide claddings have lowerrefractive indices than that of the waveguide core so thatn_(ICWRCd-z1)<n_(ICWCo-z1) and n_(ICWLCd-z1)<n_(ICWCo-z1). The verticaldirection is the direction perpendicular to the substrate plane and thehorizontal direction is the direction parallel to the substrate plane.Again, there can be one or plurality of cladding material regions, andthe four cladding regions are mentioned for the purpose of illustrationand not limitation.

In an exemplary embodiment,n_(ITWTCd-z1)=n_(ITWBCd-z1)=n_(ITWLCd-z1)=n_(ITWRCd-z1)=n_(ICWTCd), andn_(ICWTCd)=n_(ICWBCd)=n_(ICWLCd)=n_(ICWRCd) so all the cladding indicesin the tapering regions and the input connecting waveguide regions areall approximately equal. For example, these cladding regions can befilled with silicon dioxide materials with refractive index of n˜1.45.The refractive index of the waveguide core n_(ITWCo-z1) 22300 n-z 1 canbe silicon so that n_(ITWCo)=n_(ICWCo)˜3.6, where n_(ICWCo) 22200 n isthe refractive index of the waveguide core for the input connectingwaveguide.

On top of the input tapering waveguide core ITWCo 22300 starting atz1=z1ALS 22300 z 1ALS, is laid with an active layer structure ALS 22500.Typically z1ALS is before g_(ITWCo) 22300 g so that 0<z1ALS<g_(ITWCo).The active layer structure starting at z1ALS can also have an up-taperwith width tapering from narrow to wide in the direction toward the ALSstructure. The various embodiments of this active layer structure ALS22500 will be described in more detail below.

Alignment Insensitive Input Beam Coupler Structure (AI-IBCS)

(1) Broadened input region, preferably narrower then mode 3.

(2) Top taper first

(3) Close to equal width

(4) Narrow the lower one down

(5) Narrow the top one down

(6) Zigzag situation

Active Layer Structure-Beam Transport into the Structure

Bottom Side Conduction and Ohmic Contact Layer

FIGS. 16a, 16b, and 16c show exemplary embodiments of the Active LayerStructure ALS 22500, which are exemplary cross-sections at location B-B′of FIG. 15a or FIG. 15b . In ALS 22500, at least a bottom sideconduction and Ohmic contact layer BSCOC 21300 is disposed somewhereabove part of input supporting structure ISTR 21200. The layer BSCOC21300 with thickness d_(BSC) 21300 d, refractive index n_(BSC) 21300 n,a total width w_(BSC) 21300 w serves to conduct electrical current andvoltage from a contact region to an electro-active layer (ECL).

Bottom Interspaced Material Layer

There can be other bottom interspaced material layer BIM 21250 betweenlayer BSCOC 21300 and ISTR 21200, with thickness d_(BIM) 21250 d, atotal width w_(BIM) 21250 w, and refractive index n_(BIM) 21250 n. Thislayer may be electrically conducting or electrical insulating. The valueof d_(BIM) may take on zero thickness, in that case the bottominterspaced material layer BIM 21250 does not exist. The existence of abottom interspaced material layer BIM 21250 is thus optional.

Bottom Metal Contact Pads

On top and to the left side of the bottom side conduction and Ohmiccontact layer BSCOC 21300 is deposed of at least a first bottom leftmetal contact pad FBLM 21900L with thickness d_(FBLM) 21900Ld, widthw_(FBLM) 21900Lw, and length g_(FBLM) 21900Lg.

On top and to the right side of the bottom side conduction and Ohmiccontact layer BSCOC 21300 is deposed of at least a first bottom rightmetal contact pad FBRM 21900R with thickness d_(FBRM) 21900Rd, widthw_(FBRM) 21900Rw, and length g_(FBRM) 21900Rw. In an exemplaryembodiment, only either the first bottom left or the first bottom rightmetal contact pad is present. In another exemplary embodiment, pluralityof such bottom metal contact pads is present. The exact location ofthese metal contact pads can be in many other locations beside the leftor right location shown as long as the metal contact pads are inelectrical contact with the bottom side conduction and Ohmic contactlayer BSCOC 21300.

Bottom Metal Electrodes

On top of the first bottom left metal contact pad FBLM 21900L is a firstbottom left metal electrode FBLME 21120L. On top of the first bottomright metal contact pad FBRM 21900R is a first bottom right metalelectrode FBRME 21120R. In an exemplary embodiment, only either thefirst bottom left or the first bottom right metal electrode is present.In another exemplary embodiment, plurality of such bottom metalelectrodes is present. The exact location of these bottom metalelectrodes can be in many other locations beside the left or rightlocation shown as long as the bottom metal electrodes are in electricalcontact with the respective bottom metal contact pads.

Bottom Interspaced Dielectric Current Conduction Layer

On top of the center region of the layer BSCOC 21300 (i.e. region aboveor near supporting structure 21200) is deposed of a bottom interspaceddielectric current conduction layer BIDC 21350 with thickness d_(BIDC)21350 d, layer width w_(BIDC) 21350 w, and an averaged refractive indexn_(BIDC) 21350 n. The layer width w_(BIDC) is the dimension of thatlayer in a horizontal direction perpendicular to the direction of theoptical beam propagation. This layer is optional in that when thicknessd_(BIDC) 21350 d is zero, this layer does not exist.

Bottom Vertical Current Conduction Layer

On top of the bottom interspaced dielectric current conduction layerBIDC 21350 is deposed of a bottom vertical current conduction layer BVC21400 with thickness d_(BVC) 21400 d, layer width w_(BVC) 21400 w, andan averaged refractive index n_(BVC) 21400 n. The layer width w_(BVC) isthe dimension of that layer in a horizontal direction perpendicular tothe direction of the optical beam propagation.

Electro-Active Layer

On top of the bottom vertical current conduction layer BVC 21400 isdeposed of an electro-active layer EC 21500 with thickness d_(EC) 21500d, width w_(EC) 21500 w, an averaged refractive index of the entirelayer given by n_(EC) 21500 n, and an averaged absorption coefficient ofthe entire layer given by □_(EC) 21500 a. The refractive index averagingis given in a similar way as illustrated by Eq. (13). The refractiveindex n_(EC) or the optical absorption coefficient α_(EC) (α_(EC)>0means optical absorption and □_(EC)<0 means optical gain) describing thefraction of energy absorbed per unit beam propagation length of theelectro-active layer can be altered by an applied electric field, anelectric current, or either injection or depletion of carriers in theelectro-active layer. The guided optical beam in the electro-activelayer BEC 21140 in this electro-active waveguiding core structure EWCoS22600 has a propagating refractive index n_(BEC) 21140 n. While in apreferred embodiment described below, the EC layer is made ofsemiconductor, it can also be any other active material according tovarious embodiments of the current invention. For example, it can beferroelectric electro-optic material (e.g. LiNbO₃ or BaTiO₃) or organicelectro-optic or electro-absorption material, whose refractive index oroptical intensity gain and absorption coefficient α_(EC) (α_(EC)<0 meansoptical gain and α_(EC)>0 means optical absorption) can be altered underan applied electric field or electric current or optical excitation beamas is well known to those skilled in the art.

In the case of an electro-optic modulator, a small averaged increment ordecrement in the averaged refractive index of the electro-active layerEC 21500 is denoted as dn_(EC) 21500 dn so that its new averagerefractive index becomes n_(EC)(new)=n_(EC)+dn_(EC) will cause a changein the propagating refractive index n_(BEC) 21140 n by dn_(BEC) 21140 dnfrom n_(BEC) to n_(BEC)(new)=n_(BEC)+dn_(BEC) due to the overlapping ofthe optical beam energy with the material regions in which dn isnon-zero.

In the case of an optical amplifier or an electro-absorption modulator,a small averaged increment or decrement in the averaged opticalintensity absorption/gain coefficient of the electro-active layer EC21500 is denoted as dα_(EC) 21500 da so that its new average opticalintensity loss/gain coefficient becomes α_(EC)(new)=α_(EC)+dα_(EC) willcause a change in the absorption/gain coefficient α_(BEC) 21140 a of theoptical beam by dα_(EC) 21140 da from α_(BEC) toα_(BEC)(new)=α_(BEC)+dα_(BEC) due to the overlapping of the optical beamenergy with the material regions in which dot is non-zero.

Top Vertical Current Conduction Layer

On top of the electro-active layer EC 21500 is deposed of a top verticalcurrent conduction layer TVC 21600 with thickness d_(TVC) 21600 d, widthW_(TVC) 21600 w, and an averaged refractive index n_(TVC) 21600 n

Top Interspaced Dielectric Current Conduction Layer

On top of the top vertical current conduction layer TVC 21600 is deposedof a top interspaced dielectric conduction layer TIDC 21650 withthickness d_(TIDC) 21650 d, width w_(TIDC) 21650 w, and an averagedrefractive index n_(TIDC) 21650 n. This layer is optional in that whenthickness d_(TIDC) 21650 d is zero, this layer does not exist.

Top Vertical/Side Conduction and Ohmic Contact Layer

On top of the top interspaced dielectric conduction layer TIDC 21650 isdeposed of a top vertical/side conduction and Ohmic contact layer TVSCOC21700 with thickness d_(TVSC) 21700 d, width w_(TVSC) 21700 w, and anaveraged refractive index n_(TVSC) 21700 n.

Top Metal Contact Pads

In one embodiment (FIG. 16a ), on top of the top vertical/sideconduction and Ohmic contact layer TVSCOC 21700 is deposed of a firsttop right metal contact pad FTRM 21800R with thickness d_(FTRM) 21800Rd,width w_(FTRM) 21800Rw, and length g_(FTRM) 21800Rg. FTRM 21800R istypically to the right end on the top of TVSCOC 21700. In anotherembodiment (FIG. 16b ), On top of the top vertical/side conduction andOhmic contact layer TVSCOC 21700 is deposed of a first top left metalcontact pad FTLM 21800L with thickness d_(FTLM) 21800Ld, width w_(FTLM)21800Lw, and length g_(FTLM) 21800Lg. FTLM 21800L is typically to theleft end on the top of TVSCOC 21700. In as yet another embodiment (FIG.16C), on top of the top vertical/side conduction and Ohmic contact layerTVSCOC 21700 is deposed of a first top middle metal contact pad FTMM21800M with thickness d_(FTMM) 21800Md, width w_(FTMM) 21800Mw, andlength g_(FTMM) 21800Mg. FTMM 21800M is typically at the middle part onthe top of TVSCOC 21700. Middle part is the portion closest to the beamBEC 21140. This case shown by FIG. 16C is particularly applicable whenthe top vertical/side conduction and Ohmic contact layer TVSCOC 21700 isan Ohmic Transparent Conductor (OTC) or Low-Refractive-Index OhmicTransparent Conductor (LRI-OTC).

In an exemplary embodiment, only either the first top left, first topmiddle, or the first top right metal contact pad is present. In anotherexemplary embodiment, plurality of such top metal contact pads arepresent. The exact location of these top metal contact pads can be inmany other locations beside the left or right location shown as long asthe top metal contact pads are in electrical contact with the topvertical/side conduction and Ohmic contact layer TVSCOC 21700.

Top Metal Electrodes

On top of the first top left metal contact pad FTLM 21800L is a firsttop left metal electrode FTLME 21130L. On top of the first top middlemetal contact pad FTMM 21800M is a first top middle metal electrodeFTMME 21130M. On top of the first top right metal contact pad FTRM21800R is a first top right metal electrode FTRME 21130R. In anexemplary embodiment, only either the first top left, first top middle,or the first top right metal electrode are present. In another exemplaryembodiment, plurality of such top metal electrodes is present. The exactlocations of these top metal electrodes can be in many other locationsbeside the left or right location shown as long as the top metalelectrodes are in electrical contact with the respective top metalcontact pads.

Electro-Active Waveguiding Core Structure and Central Waveguide CoreLayer

A layer or several layers that are in spatial proximity to theelectro-active layer EC 21500 form an electro-active waveguiding corestructure EWCoS 22600 at least a portion of it contains a centralwaveguide core layer WCo 22600Co. For the purpose of illustration andnot limitation, a particular exemplary embodiment of an electro-activewaveguiding core structure EWCoS 22600 is formed by the bottom verticalcurrent conduction layer BVC 21400, the top vertical current conductionlayer TVC 21600, and the electro-active layer EC 21500 as shown by FIG.17. However, this electro-active waveguiding core structure can also beformed by any other layer or collection of layers as long as it is inspatial proximity to the electro-active layer so that the optical beamguided by it will have a reasonable amount of optical energy in theelectro-active layer. The central waveguide core layer WCo 22600Co hasan averaged refractive index n_(WCo) 22600Con higher than the refractiveindex of most of the materials surrounding it. As is known to thoseskilled in the art, a waveguide core only needs its refractive index tobe generally higher than the refractive indices of most of itssurrounding materials in order to confine and guide optical beam (e.g.one example of such waveguides is commonly known as “rib waveguide” or“ridge waveguide”). For illustration and not limitation, in an exemplaryembodiment, the central waveguide core layer WCo 22600Co is theelectro-active layer EC 21500. As is known to those skilled in the art,the central waveguide core layer WCo 22600Co can also be in part oflayer 21400 or layer 21600.

The central waveguide core layer WCo 22600Co has an averaged refractiveindex n_(WCo) 22600Con higher than the refractive indices of most itssurrounding and confines optical beam energy of beam BEC 21140, calledthe beam electro-active or beam EC, in the vertical and horizontaldirections so that the peak of the beam intensity is within or near thecentral waveguide core layer 22600Co, and the optical beam is said to bea guided optical beam. The guided optical beam BEC 21140 in thiselectro-active waveguiding core structure EWCoS 22600 has a propagatingrefractive index n_(BEC) 21140 n that is smaller than the materialrefractive index of the central waveguide core layer n_(WCo) 22600Con sothat n_(BEC)<n_(WCo). This criterion can be taken as the definition ofthe material region that made up the waveguide core (i.e. it is theregion in which the material refractive index is higher than the beampropagating refractive index n_(BEC)).

Electro-Active Waveguide Core and Cladding Regions for Beam EC

As is known to those skilled in the art, the entire electro-activewaveguide core region for beam EC is the material region occupied by thebeam EC, BEC 21140, in which the refractive index of the material isgenerally higher than n_(BEC) 21140 n. As is also known to those skilledin the art, the electro-active waveguide cladding regions for beam ECare the material regions occupied by the beam in which the refractiveindex of the material is generally lower than n_(BEC). For the purposeof discussion, one may take the electro-active waveguiding corestructure EWCoS 22600 mentioned above as defined by this electro-opticwaveguide core region.

Thus, the electro-active layer 22500 may be or may not be part of thewaveguide core region for beam guided in the EC-layer BEC 21140 as longas the electro-active layer 22500 is in spatial proximity to thewaveguide core region for EC-layer beam BEC 21140 so that a reasonableamount of the beam's optical energy is in the electro-active layer. Evenif the electro-active layer 22500 is part of the waveguide core region,it is not necessarily the entire waveguide core region for EC-layer beamBEC 21140.

Beam Transport to Electro-Active Waveguide Core Structure

Most of the input optical beam energy of input beam IBM 22140 istransported from input tapering waveguide core ITWCo 22300 to theelectro-active waveguide core structure EWCoS 22600, through the inputtapering waveguide region between z1=z1ALS 22300 z 1ALS and z1=g_(ITWCo)22300 g, where the tapering waveguide core width w_(ITWCo-z1) 22300 w-z1 varies to a value of w_(ITWCo-g) at z1=g_(ITWCo) 22300 g from itsvalue at z1=z1ALS 22300 z 1ALS (it can be the same value as, smallerthan, or larger than its value at z1=z1ALS 22300 z 1ALS). In a preferredembodiment, for the purpose of illustration and not limitation, this isenabled by reducing the tapering waveguide core width from a valueapproximately equal to or larger than half the optical wavelength in thewaveguide core given by λ_(bm)/(2*n_(ITWCo)), to well below half theoptical wavelength in the waveguide core given by λ_(bm)/(2*n_(ITWCo))so that W_(ITWCo-g)<<λ_(bm)/(2*n_(ITWCo)), where * is numericalmultiplication. More exactly, it is reduced from a width that is a widththat enables the optical energy to be well confined in the waveguidecore ITWCo 22300 just before it enters the ALS 22500 to a width (afterit enter the ALS 22500) such that the optical energy is no longer wellconfined in the waveguide core ITWCo 22300 after it enters the ALS 22500(the width for no longer well-confined is defined by the beamconfinement after the taper waveguide core enters ALS 22500). Wellconfined means over 50% of the beam energy is in the waveguide coreITWCo 22300. Depending on the application situation, this can mean asmaller width (e.g. if the refractive index of the EC layer isapproximately equal to or lower than the refractive index of the inputtapering waveguide). It can also maintain the same width or even go to alarger width (e.g. if the refractive index of the EC layer is higherthan the refractive index of the input tapering waveguide).

After the energy is transported to electro-active waveguide corestructure EWCoS 22600 that contains the electro-active layer EC 21500,the optical beam is denoted as optical beam in the electro-active regionor EC layer beam, BEC 21140.

Output Connecting Waveguide

Output connecting waveguide core OCWCo 28200. The output connectingwaveguide core OCWCo 28200 is fabricated on an outputconnecting-waveguide bottom cladding material OCWBCd 28200B disposed ona substrate SUB 21100 (FIG. 18).

In one exemplary embodiment shown in FIG. 19, illustrating an exemplaryembodiment of the cross section at location C-C′ of FIG. 18, to the topof the output connecting waveguide core is further deposed with outputconnecting waveguide top cladding material OCWTCd 28200T (FIG. 19).Bottom refers to direction closer to the substrate and top refers todirection away from the substrate. To the left of the input connectingwaveguide core is deposed with output connecting waveguide left claddingmaterial OCWLCd 28200L and to the right of the waveguide core isdisposed with output connecting waveguide right cladding material OCWRCd28200R. Right or left is taking the direction of beam propagation as thefront direction and right or left means relative to this frontdirection. The division of the cladding into four different materialregions as illustrated by FIG. 19 is for the purpose of discussion andnot limitation as these can be all the same materials or there can bemore than 4 material regions forming plurality of different materialregions as long as these regions act as waveguide cladding materialswith effective refractive index generally smaller than the effectiverefractive index of the waveguide core OCWCo 28200 so that the lightbeam power is confined near the region of the waveguide core OCWCo28200, as is well known to those skilled in the art. The claddingmaterial is a general designation that can include “air” or “vacuum” ortransparent dielectric material as the material.

Output Connecting Waveguide Region

The output connecting waveguide core OCWCo 28200 is made up of amaterial or mixture of materials with an averaged material refractiveindex n_(OCWCo) 28200 n, has a thickness d_(OCWCo) 28200 d, and widthW_(OCWCo) 28200 w. Let the refractive index of the bottom inputconnecting-waveguide bottom cladding material be n_(OCWBCd) 28200Bn. Letthe refractive index of the top cladding material OCWTCd 28200T ben_(OCWTCd) 28200Tn, the refractive index of the left cladding materialOCWLCd 28200L be n_(OCWLCd) 28200Ln, and the refractive index of theright cladding material OCWRCd 28200R be n_(OCWRCd) 28200Rn. Thewaveguide core 28200 and the claddings 28200T, 28200B, 28200R, 28200L,together forms output connecting waveguide OCWG 28200WG.

The vertical confinement of the optical beam, called output optical beamOBM 28140, in the output connecting waveguide is due to therefractive-index difference between the top and bottom waveguidecladdings and the waveguide core and the claddings generally have lowerrefractive indices than that of the waveguide core so thatn_(OCWTCd)<n_(OCWCo) and n_(OCWBCd)<n_(OCWCo). The horizontalconfinement of the optical beam is due to the refractive-indexdifference between the left and right waveguide claddings and thewaveguide claddings generally have lower refractive indices than that ofthe waveguide core so that n_(OCWRCd)<n_(OCWCo) andn_(OCWLCd)<n_(OCWCo). The vertical direction is the directionperpendicular to the substrate plane and the horizontal direction is thedirection parallel to the substrate plane. The output beam has apropagating refractive index given by n_(OBM) 28140 n.

An important quantity in terms of waveguide mode confinement is therefractive index contrast between the averaged refractive index of thewaveguide core and its immediate surrounding cladding materials calledthe refractive-index difference n_(Rd) defined by n_(Rd) ²=□n_(Co)²−n_(Cd) ²), where n_(Co) is the refractive index of the waveguide core(e.g. n_(Co)=n_(OCWCo)) and n_(Cd) is the refractive index of thewaveguide cladding (e.g. n_(Cd)=n_(OCWBCd) or n_(OCWTCd) or n_(OCWRCd)or n_(OCWLCd)) or an averaged of them thereof given by n_(aOCWCd) 28200aCdn. The refractive-index averaging is more accurately done as averagedof its square values which are their dielectric constant □=n², asillustrated by Eq. (13).

Likewise the waveguide core can also be made up of one or plurality ofmaterials, and n_(OCWCo)=n_(aOCWCo) can also be an averaged refractiveindex of the “m” materials with slightly different refractive indicesn_(OCWCo1), n_(OCWCo2), n_(OCWCo3) . . . n_(OCWCo m), that made up ofthe waveguide core materials.

Output Beam Coupler Structure (OBCS) Region

FIG. 20 shows the output beam coupler structure (OBCS) comprises atleast a tapering waveguide section (preferably tapering from wide tonarrow but can also maintain the same width or taper from narrow to wideas will be elaborated below) connected to the input waveguide.Optionally, the active layer structure ALS on top of the output taperingwaveguide section can also be tapering in the form of an up taper(preferably tapering from narrow to wide in the direction toward theactive layer structure ALS, but can also maintain the same width ortaper from wide to narrow). See for example FIG. 20 b.

Specifically, the energy of the electro-active beam BEC 21140 in theelectro-active waveguiding core structure EWCoS 22600 is coupledefficiently to the output optical beam IBM 28140 energy in the outputconnecting waveguide core OCWCo 28200 via propagating through an outputconnecting-waveguide taper region. The output connecting-waveguide taperregion has a output tapering waveguide core OTWCo 28300. The outputtapering waveguide core at a location z2 is denoted by OTWCo-z2 28300 z2 (FIG. 19a ), at which the width of the tapering waveguide core isdenoted by w_(OTWCo-z2) 28300 w-z 2. The distance or location parameterz2 is given by the distance measured from the beginning point of thetaper that is at a point along the output connecting waveguide coreOCWCo 28200, typically outside (or away from) the ALS region. It ischanged from its input value at z2=0 w_(OTWCo-z2=0) 28300 w-z 2=0 ofw_(OTWCo-z2=0)=w_(OCWCo) 28200 to another width (that can be the samewidth) at z2>0 w_(OTWCo-z2>0) 28300 w-z 2>0. The thickness of thetapering waveguide core is d_(OTWCo-z1) 28300 d-z 2 and its refractiveindex is n_(OTWCo-z1) 28300 n-z 2. Typically, though not always,d_(OTWCo-z2) 28300 d-z 2 and n_(OTWCo-z2) 28300 n-z 2 are constant valuein z2 so we can drop the z2 designation with d_(OTWCo-z1)=d_(OTWCo-z2)28300 d and n _(OTWCo-z2)=n_(OTWCo) 28300 n. In a preferred embodiment,d_(OTWCo-z2) is about the same value as d_(OCWCo). The tapering may besuch that w_(OTWCo-z2) is a linear function of z2 or quadratic functionof z2 (i.e. depending on z2²), but can also be of any curvilinearfunction of z2. Let g_(OTWCo) denote the total length of this taperingwaveguide.

The end of the taper at z2=g_(OTWCo) 28300 g, typically inside (ortoward) the ALS region at which the width of the waveguide core isw_(OTWCo-g) 28300 w-g, is connected to output supporting structure OSTR29200 that may be a continuation from and in some way physicallyconnected to the input supporting structure ISTR 21200 or may beindependent of it. While illustrated as a line that is continuation ofthe connecting waveguide material with a narrow width and air or otherlow refractive index materials surrounding its side, the outputsupporting structure can be random dots or any shape of small amount ofany materials that have a low “effective refractive index” or small“averaged refractive index” (e.g. as defined by Eq. (13)) as is known tothose skilled in the art, comparing to the refractive index of thewaveguide core n_(WCo) 22600Con in the electro-active waveguiding corestructure EWCoS 22600, resulting in an effective averaged refractiveindex n_(aOSTR) 29200 na for this entire layer of supporting structure.The output supporting structure OSTR 29200 may continue to guide wave orjust acts as a supporting structure, depending on application scenarios.

In an exemplary embodiment, the output supporting structure OSTR 29200is a narrow line. In that particular case, we can describe it as havinga width w_(OSTR) 29200 w, thickness d_(OSTR) 29200 d, and lengthg_(OSTR) 29200 g. The length g_(OSTR) 29200 g may be zero. In that case,output supporting structure OSTR 29200 does not exist (the thin ALS filmcan still be supported by its corners or sides, but not directly below).In a preferred embodiment, d_(OSTR) is about the same value asd_(OCWCo). At some point the output supporting structure OSTR 29200merges with the input supporting structure ISTR 21200 and thus ISTR21200 and OSTR 29200 may be used interchangeably.

In region outside the ALS region, the vertical confinement of theoptical beam along the taper is due to the refractive-index differencebetween the tapering waveguide core and the top and bottom taperingwaveguide claddings at the location z defined above: OTWTCd-z2 28300T-z2(refractive index n_(OTWTCd-z1) 28300Tn-z2) and OTWBCd-z1 28300B-z2(refractive index n_(OTWBCd-z1) 28300Bn-z2) and the waveguide core andthe claddings generally have lower refractive indices than that of thewaveguide core so that the refractive index n_(OTWTCd-z2)<n_(OTWCo-z2)and n_(OTWBCd-z2)<n_(OTWCo-z2). The horizontal confinement of theoptical beam is due to the refractive-index difference between the leftand right waveguide claddings at z2: OTWLCd-z2 28300L-z2 (refractiveindex n_(OTWLCd-z2) 28300Ln-z2) and OTWRCd-z2 28300R-z2 (refractiveindex n_(OTWRCd-z2) 28300Rn-z2), and the waveguide claddings generallyhave lower refractive indices than that of the waveguide core so thatn_(OCWRCd-z2)<n_(OCWCo-z2) and n_(OCWLCd-z2)<n_(OCWCo-z2). The verticaldirection is the direction perpendicular to the substrate plane and thehorizontal direction is the direction parallel to the substrate plane.Again, there can be one or a plurality of cladding material regions andthe four cladding material regions are mentioned for the purpose ofillustration and not limitation.

In an exemplary embodiment,n_(OTWTCd-z2)=n_(OTWBCd-z2)=n_(OTWLCd-z2)=n_(OTWRCd-z2)=n_(OCWTCd) andn_(OCWTCd)=n_(OTWRCd-z2)=n_(OCWRCd) so all the cladding indices in thetapering regions and the input connecting waveguide regions are allapproximately equal. For example, these cladding regions can be filledwith silicon dioxide materials with refractive index of n˜1.45. Therefractive index of the waveguide core n_(OTWCo-z2) 28300 n-z 2 can besilicon so that n_(ITWCo)=n_(OCWCo)˜3.6, where n_(OCWCo) 28200 n is therefractive index of the waveguide core for the input connectingwaveguide.

On top of the tapering waveguide core OTWCo 28300 starting at z2=z2ALS28300 z 2ALS, is laid with an active layer structure ALS 22500.Typically z2ALS is before g_(OTWCo) so that 0<z2ALS<g_(OTWCo). An“active layer” is a material layer that can give optical gain or opticalabsorption or change in the refractive index. The various embodiments ofthis active layer structure ALS 22500 have already been described above.

Active Layer Structure-Beam Transport from the Structure to Output

Most of the output optical beam energy of beam OBM 28140 is transportedto output tapering waveguide core OTWCo 28300 from the electro-activewaveguiding core structure EWCoS 22600, through the output taperingwaveguide region that typically lies inside the ALS region, betweenz2=g_(OTWCo) 28300 g and z2=z2ALS 28300 z 2ALS, where the taperingwaveguide core width w_(OTWCo-z1) 28300 w-z 2 varies from a value ofw_(OTWCo-g) at z2=g_(OTWCo) 28300 g to a value of w_(OTWCo-z2ALS) atz2=z2ALS 28300 z 2ALS (it can be the same value as, smaller than, orlarger than its value at z2=z2ALS 28300 z 2ALS). In a preferredembodiment, for the purpose of illustration and not limitation, this isenabled by changing the tapering waveguide core width at z2=z2ALS 28300z 2ALS from a value approximately equal to or larger than half theoptical wavelength in the waveguide core given by λ_(bm)/(2*n_(OTWCo)),to well below half the optical wavelength in the waveguide core atz2=g_(OTWCo) 28300 g given by λ_(bm)/(2*n_(OTWCo)) so thatW_(OTWCo-g)<<λ_(bm)(2*n_(OTWCo)), where * is number multiplication. Moreexactly, it is increased from a narrow width (in a region inside the ALS22500) for which the optical energy is not well confined in thewaveguide core OTWCo 28300 (the width for not well-confined is definedby the beam confinement in the waveguide core OTWCo 28300) to a widerwidth that enables the optical energy to be well confined in thewaveguide core OTWCo 28300 just around when it exits the ALS 22500region.

Well confined means over 50% of the beam energy is in the waveguide coreOTWCo 28300. Depending on the application situation, this can mean asmaller width (e.g. if the refractive index of the EC layer isapproximately equal to or larger than the refractive index of the inputtapering waveguide). It can also maintain the same width or even go to alarger width (e.g. if the refractive index of the EC layer is higherthan the refractive index of the input tapering waveguide).

After the energy is transported from the electro-active waveguiding corestructure EWCoS 22600 that contains the electro-active layer EC 21500down to the output taper at z2=Z2ALS and further propagated to the taperstarting location at z2=0 where the taper core width is w_(OTWCo-z2=0)28300 w-z 2=0 and w_(OTWCo-z2=0)=w_(OCWCo) 28200, the optical beam isdenoted as output optical beam or beam OBM 28140. Note that at z2=0, theoutput tapering waveguide core OTWCo-z2 28300 z 2 is joined to outputconnecting waveguide core OCWCo 28200.

Length of Active Layer Structure

The active layer structure ALS runs a length from the input taperingwaveguide core ITWCo 22300 at z1=z1ALS to the output tapering waveguidecore OTWCo 28300 at z2=z2ALS. Along the ALS structure, the distance fromz1=z1ALS is parameterized as coordinate z. Location z thus measures aspecific location along the length of the active layer structure ALS22500. The total length of ALS 22500 from z1=z1ALS to z2=z2ALS is calledthe structure length of the modulator SL_(mod) 22550. Coordinate z endsat z2=z2ALS at which z=SL_(mod).

Along z, the various widths and thicknesses of each of the layers in theALS may vary and do not necessarily have to stay constant. As is knownto those skilled in the art, such variation in widths and thicknesseswill not affect the general performance of the modulator. In addition,there may be more or fewer layers in the ALS other than specified aslong as the functionalities of those layers specified are equivalentlyperformed by the additional or fewer layers. As is known to thoseskilled in the art, such variations will not affect the generalperformance of the modulator. Hence, the various ALS structuralvariations as described above are for the purpose of illustration andnot limitation.

Active Layer Structure-Electro Active Layer

The active material ACM 21500M in EC layer 21500 can be any activematerial as is known to those skilled in the art in which an appliedelectric field will change its refractive index or optical absorption oroptical gain in at least a portion of the material. The entire structuredescribed above can be used with any active material in layer 21500.While we illustrate a particular semiconductor active material below, itis only one of the many possibilities, and is to illustrate a particularpreferred embodiment of the intensity or phase modulator in the presentinvention. They are not meant to limit the scope of the invention.

Semiconductor EC Material Layer

As noted, the electro-active layer EC need not be made of semiconductormaterials. As an exemplary embodiment, in the case for which theelectro-active layer is made of semiconductor based material as shown inFIG. 21, the structure in the electro-active layer could comprise a PNjunction 21500PN in which a first P-layer 21500LP₁ with P-type carrierdopant or called P-dopant (i.e. the resulting carriers from the dopantatoms are holes) and dopant density given by P₁, 21500P₁ is verticallyphysically connected (vertical means in a direction perpendicular to thesubstrate plane: horizontal means in a direction parallel to thesubstrate plane) to a first N-layer 21500LN₁ with N-type carrier dopantor called N-dopant (i.e. the resulting carriers from the dopant atomsare electrons) and dopant density given by N₁ 21500N₁. Depending on theapplication situations, the electro-active layer may be the entire PNstructure itself or may be part of the PN structure or may be justelectrically connected to the PN structure.

Alternatively, the electro-active layer structure could also comprise aPqN junction 21500PqN in which a first P-layer 21500LP₁ with P-dopantand dopant density given by P₁ 21500P₁ is connected to a middle q-layerwith either N dopant, P dopant, or being intrinsic “I” (i.e. commonlymeans with very low dopant or no dopant or being an Intrinsicsemiconductor material) labeled as 21500MLqm (e.g. it will be labeled as21500MLI_(m) if it is intrinsic (i.e. undoped or being an intrinsicsemiconductor material), 21500MLN_(m) if it is N doped, and 21500MLP_(m)if it is P-doped; m is an integer to sub-label the layer number anddopant density given by Mq_(m), 21500Mq_(m) (e.g. it will be labeled as21500MI₁ if it is intrinsic I₁, 21500MN₁ if it is N₁ doped, and 21500MP₁if it is P₁-doped), and the middle q-layer is further connected to afirst N-layer 21500LN₁ with N₁-dopant and dopant density given by N121500N₁. This middle q-layer may be made up of plurality of one or moredoped layers 21500MLq1, 21500MLq2, . . . 21500MLqT, where T is aninteger specifying the number of layers. Depending on the applicationsituations, the electro-active layer may be the entire PqN structureitself or may be part of the PN structure or may be just electricallyconnected to the PqN structure.

Further alternatively, the electro-active layer structure could comprisea NqN junction 21500NqN in which a first N-layer 21500LN₁ with N-dopantand dopant density given by N₁ 21500N₁ is connected to a middle q-layerwith either N dopant, P dopant, or being intrinsic “I” (i.e. commonlymeans with very low dopant or no dopant or being an intrinsicsemiconductor material) labeled as 21500MLqm (e.g. it will be labeled as21500MLI_(m) if it is intrinsic (i.e. undoped or being an Intrinsicsemiconductor material), 21500MLN_(m) if it is N doped, and 21500MLP_(m)if it is P-doped: m is an integer to sub-label the layer number) anddopant density given by Mq_(m), 21500Mq_(m) (e.g. it will be labeled as21500MI₁ if it is intrinsic I₁, 21500MN₁ if it is N₁ doped, and 21500MP₁if it is P₁-doped), and the middle q-layer is further connected to asecond N-layer 21500LN₂ with N₂-dopant and dopant density given by N₂21500N₂. This middle q-layer may be made up of plurality of one or moredoped layers 21500MLq1, 21500MLq2, . . . 21500MLqT, where T is aninteger specifying the number of layers. Depending on the applicationsituations, the electro-active layer may be the entire NqN structureitself or may be part of the NqN structure or may be just electricallyconnected to the NqN structure.

Further alternatively, the electro-active layer structure could comprisea XqY junction 21500NqN in which a first X-layer 21500LX₁ is connectedto a middle q-layer with either N dopant, P dopant, or being intrinsic“I” (i.e. commonly means with very low dopant or no dopant or being anintrinsic semiconductor material) labeled as 21500MLqm (e.g. it will belabeled as 21500MLI_(m) if it is intrinsic (i.e. undoped or being anIntrinsic semiconductor material), 21500MLN_(m) if it is N doped, and21500MLP_(m) if it is P-doped; m is an integer to sub-label the layernumber) and dopant density given by Mq_(m), 21500Mq_(m) (e.g. it will belabeled as 21500MI₁ if it is intrinsic I₁, 21500MN₁ if it is N₁ doped,and 21500MP₁ if it is P₁-doped), and the middle q-layer is furtherconnected to a second Y-layer 21500LY₁. This middle q-layer may be madeup of plurality of one or more doped layers 21500MLq1, 21500MLq2, . . .21500MLqT, where T is an integer specifying the number of layers.Depending on the application situations, the electro-active layer may bethe entire XqY structure itself or may be part of the XqY structure ormay be just electrically connected to the XqY structure. In the above,X₁ and Y₁, each may either be N-doped, P-doped, or being an intrinsic“I” semiconductor, and X₁ and Y₁ can be doped differently with differentdopant type.

The P and N dopants may have spatially varying profiles in terms oftheir doping density (number of dopant carriers per unit volume) and theprofiles may vary from one application to another. While there arevarious mode of operation for the active material, a commonly used modeis to apply reverse bias voltage V_(R) 21500VR across the abovementionedPN or PqN layers (with negative voltage on the P side and positivevoltage on the N side), so that an electric field E_(EC) is generated togo across part of the EC layer 21500.

Depending on the application, for the abovementioned PN or PqN or NqN orXqY structure, the N₁ doped layer may be above or below the P₁-dopedlayer (above means further away from the substrate and below meanscloser to the substrate). The EC layer 21500 may have quantum wells inthe structure, typically in the q layer or close to the PN junction. Atleast one of the first P-layer, first N-layer, or the middle q-layercontains at least one quantum well. One or more quantum wells can alsobe in both the first P-layer and first N-layer or in all the threelayers: first P-layer, first N-layer, and middle q-layer or just in themiddle q-layer. The quantum wells can be strained, unstrained, ordouble-well or multiple-well quantum wells as is known to those skilledin the art. It can also have no quantum well.

As an exemplary illustration, without quantum wells and without carrierdoping in the q layer, in the case of EO modulation, the mainelectro-optic phase shift will be due mainly to linear electro-optic(LEO) effect. If q has carrier doping (N or P) then it will add plasma(PL) and bandfilling (BF) effect. If q layer has quantum wells, thenquantum-confined stark effect (QCSE) will be added to enhance the EOphase shift. If the PqN is forward bias, then a lot of carriers will beinjected into the q layer, causing refractive index change due tocarrier injections or depletions. This may give significant phase shiftsin the electro active (EC) material layer just due to PL and BF effects.However such modulator will be slow as removing the carrier is a slowprocess, typically at nano second speed or slower (e.g. <1 GHz). Inorder to go to high modulation frequency (e.g. >1 GHz), typicallyrevised biased is applied. In that case the electric field in the qlayer will cause carrier depletion which will also give rise to PL or BFeffects, and the electric field will cause LEO effect (with or withoutquantum wells) and QCSE also (with use of quantum wells). The PL, BF,QCSE can also cause the absorption coefficient α_(EC) to change(α_(EC)>0 gives optical absorption) resulting in electro-absorptionmodulation, depending on the operating wavelength. As is known to thoseskilled in the art, for electro-absorption modulation, the operatingoptical wavelength is typically at relatively close to the band edge(edge of the material or quantum-well bandgap). For electro-opticmodulation, the operating optical wavelength is typically at relativelyfar from the band edge (edge of the material or quantum-well bandgap).With carrier injection, it may also give population inversion resultingin optical gain (α_(EC)<0 gives optical gain) and hence giving rise tooptical gain induced optical intensity or phase modulation. Theseeffects due to change in α_(EC) will result in electro-absorption orgain based modulators, instead of electro-optic modulators, and are aparticular embodiment of the present invention.

The averaged incremental change in the refractive index dn_(EC) 21500 dnor change in the optical intensity loss/gain coefficient dα_(EC) 21500da of at least part of the material in the semiconductor electro-activelayer 21500 can be caused by an applied electric field E_(EC) 21500E, anelectric current Cα_(EC) 21500C, or either injection or depletion ofcarriers in the electro-active layer 21500 (note dn_(EC) 21500 dn is notthe same as n_(EC) 21500 n, which is the averaged refractive index ofthe entire EC layer 21500 when there is no field).

Voltage and Current Conduction to the Electro-Active Layer

As shown in FIG. 21, the semiconductor electro-active layer EC 21500 iselectrically connected to the top vertical current conduction layer TVC21600 on the top side and is electrically connected to the bottomvertical current conduction layer BVC 21400 on the bottom side. Bottomside is the side nearer to the substrate 21100. Two materials areelectrically connected if an electric current can be passed between thetwo materials with a total electrical resistance times area lower thanabout 10 Ω-cm² so that for 10,000 μm² area, the total contact resistanceis less than about 100,000Ω=100 kΩ (10,000 m² area is the area for arelatively large 5 mm-long, 2 μm-wide modulator device).

The Use of Forward-Biased PN Junction or Tunnel PN Junction to ReduceP-Dopant Optical Loss

As an illustration but not limitation, for the EC 21500 layer, if its Nlayer is below the P layer, then the bottom vertical current conductionlayer BVC 21400 can be (not always) an N-doped semiconductor materialand the top vertical current conduction layer TVC 21600 can be (notalways) a P-doped semiconductor material to enable easy currentconduction without significant voltage dropped. The problem is thatP-doped material has much higher (typically 10 times) electricalresistance and optical absorption than that of N-doped material at thesame dopant density.

As will be noted below, this can be addressed as with use of a“forward-bias PN junction”, it is possible to make electrical connectionto region of opposite dopant type without significant voltage droppedand that could have certain advantages. We will refer to this as a“PN-changing PN junction” (labeled as PNCPN junction) to distinguish itfrom the PN junction inside EC layer 21500. Such PNCPN junction willconduct current or voltage when it is forward biased. Note that if sucha PN-changing PN junction has highly doped P and N layers, it can alsobe conducting electricity even when it is under a reverse bias. In thatcase we called it a tunnel junction as the current conduction isdepending of some type of carrier tunneling across the reverse-biasedjunction, as is well known to those skilled in the art. Then in thatcase, it can be used for when the PN junction inside EC layer 21500 PNjunction is either reverse or forward biased. Thus, when we call it“PN-changing PN junction”, it will be generally referred to when the PNlayer involved is either forward bias and conducting current or when itis highly doped and reverse biased but acts as a current-conductingtunneling junction.

For example, as shown in FIG. 22a , suppose in the abovementioned PN orPqN or XqY structure, the N doped layer is below the P-doped layer, thenin that case the top vertical current conduction layer TVC 21600 canalso be an N₂-doped (subscript 2 is just to label doping in this layer)semiconductor material in contact with the top layer of EC layer 21500that is P₁ doped (called layer 21500LP₁), resulting in a PNCPN junctionbetween layer 21600 and 21500 labeled as 21600-21500PNCPN.

Alternatively, as shown in FIG. 22b , layer TVC 21600 may start with aP-doped layer (called layer 21600LP₂) with a dopant density of P₂,21600P₂, connecting to the top P-doped layer of layer EC 21500 withdopant density P₁, and the P-doped layer 21600LP₂ is also connected toanother layer that is N doped (called layer 21600LN₂) with dopantdensity N₂, 21600N₂, resulting in a PNCPN junction in layer 21600labeled as 21600PNCPN. Then an applied voltage with negative voltageapplied to the N₂-doped layer TVC 21600 will give a forward bias acrossthe N₂P₂ junction and transmit the voltage to the top P₁ doped layer ofEC layer 21500 that forms part of the P₁q N₁ structure, resulting inreverse bias across the P₁q N₁ junction in EC layer 21500. In that case,the N₂P₂ layers act as a PN-changing PN junction. Likewise if the P₁q N₁junction on EC 21500 layer has P₁ at the bottom connecting to anN₂-doped layer BVC 21400, then one has a PNCPN junction between 21400and 21500 labeled as 21500-21400PNCPN junction forming the structureN₂P₁q N₁ (see FIG. 22a ); or one can have a PNCPN junction inside 21400labeled as 21400PNCPN junction forming the structure. N₁ P₂P₁q N₁ (seeFIG. 22b ).

The reason to effectively change the P-doped to N-doped layer via suchP-N changing PN junction is because an N-doped layer typically can bedoped to have a much lower electrical resistance than P-dopedsemiconductor material for two reasons: (1) the dopant density for Ndopant typically can be higher than that of P dopant; (2) even at thesame dopant density, the electrical conductivity of N doped material cantypically be higher than that of P doped material by about 10 times.Note that, as is also well known to those skilled in the art, N-dopedsemiconductor material also typically has a much lower opticalabsorption than P-doped semiconductor material even if the N-typematerial is doped to the same electrical resistance as a P-type material(typically can be about 10 times lower in optical absorption).

This enables the use of highly N-doped layer with low electricalresistance for layer 21300, 21350, and 21400 from the bottom half up and21700, 21650, and 21600 from the top half down, thereby substantiallylowering the series electrical resistance of the modulator structure.Low series electrical resistance will give high modulation frequency.

The example above is for the purpose of illustration and not limitation.For example, the PqN or PN junction in the electro-active layer EC 21500may have P-doped side at the top, instead of the bottom, and aPN-changing PN junction is used so that the top layers can becomeN-doped materials. There are thus various variations in the use of thePN-changing PN junction as shall be obvious to those skilled in the art.

Top Vertical/Side Conduction and Ohmic Contact Layer

The top vertical/side conduction and Ohmic contact layer TVSCOC 21700with thickness d_(TVSC) 21700 d and width w_(TVSC) 21700 w is alsoelectrically connected at its bottom to the top vertical currentconduction layer TVC 21600 through the top interspaced dielectricconduction layer TIDC 21650, and at its top to the top left/middle/rightmetal contact pad FT(L/M/R)M 21800(L/M/R) or any top metal contact padFTXM 21800X (X refers to any of the plurality of top metal contactpads).

Upper and Lower Waveguide Claddings of Active-Layer Structure

In one embodiment, the top vertical/side conduction and Ohmic contactlayer TVSCOC 21700, with an averaged refractive index n_(TVSC) 21700 n,forms part of an top electro-active waveguide cladding 22600TCd forwhich n_(TVSC) 21700 n is smaller than the refractive index n_(WCo)22600Con of the central waveguide core 22600Co. In one exemplaryembodiment, TVSCOC 21700 is a low-refractive-index Ohmic transparentconductor (LRI-OTC) (see illustration in FIG. 16)

The top interspaced dielectric conduction layer TIDC 21650 with anaveraged refractive index n_(TIDC) 21650 n, in another exemplaryembodiment also forms part of a top electro-active waveguide cladding22600TCd for which n_(TIDC) 21650 n is smaller than the refractive indexn_(WCo) 22600Con of the central waveguide core 22600Co.

In as yet another embodiment, the top electro-active waveguide claddingis formed by an air or dielectric region (e.g. the dielectric regionTDMR 21810 in FIG. 24 described later below) above layer TVSCOC 21700.

In one embodiment, part of a bottom electro-active waveguide cladding22600BCd, in the case where the width w_(ISTR) of the input supportstructure ISTR 21200 is narrow, may be made up of the input supportstructure ISTR 21200 below the bottom vertical current conduction layerBVC 21400 plus the cladding materials to its left, and right as follows:The input support structure ISTR 21200 is made up of a material ormixture of materials with a material refractive index n_(ISTR) 21200 n.

In a preferred embodiment, for the purpose of illustration and notlimitation, typically the input connecting-waveguide core ICWCo 22200,the input tapering waveguide core ITWCo 22300, and the input supportstructure ISTR 21200 all have a similar bottom, left and right claddingmaterials, though they can also have different cladding materials. Forthe input supporting structure ISTR 21200, let the refractive index ofthe left cladding material ISTRLCd 21200L be n_(ISTRLCd) 21200Ln, andthe refractive index of the right cladding material ISTRRCd 21200R ben_(ISTRRCd) 21200Rn. The supporting structure 21200, the left claddings21200L, and right cladding 21200R, together forms a material region withan effective layer averaged refractive index n_(laISTR) 21200 nla thatis a weighted average of n_(ISTR) 21200 n, n_(ISTRLCd) 21200Ln, andn_(ISTRRCd) 21200Rn, similar to the computation of averaged refractiveindex given by equation Eq. (13). The weighting for the averaging isdepending on the distribution of the beam energy for guided beam BEC21140 inside these material regions similar to that given by Eq. (13).The layer averaged material refractive index n_(laISTR) 21200 nlaexperienced by the guided beam BEC 21140 in regions 21200, 21200L,21200R, is typically smaller than the refractive index n_(WCo) 22600Conof the central waveguide core 22600Co. In that case, they form part ofthe bottom electro-active waveguide cladding 22600BCd.

However, in the case where the width W_(ISTR) of the input supportingstructure ISTR 21200 is relatively wide, part of a bottom electro-activewaveguide cladding 22600BCd will be made up mainly of the bottomcladding ISTRBCd 21200B below the input support structure ISTR 21200with an averaged refractive index n_(ISTRBCd) 21200Bn, which as part ofthe embodiment would be filled with materials with n_(ISTRBCd) smallerthan the refractive index n_(WCo) 22600Con of the central waveguide core22600Co. In that case, substantial optical energy can be in the inputsupporting structure STR 21200, and the input supporting structure STR21200 may become part of the waveguide core for beam BEC 21140.

In another embodiment, part of a bottom electro-active waveguidecladding 22600BCd may also be made up of the bottom interspaced materiallayer BIM 21250 (if it exists) with refractive index n_(BIM) 21250 n. Inone exemplary embodiment, BIM 21250 is a low-refractive-index Ohmictransparent conductor (LRI-OTC).

In as yet another embodiment, part of a bottom electro-active waveguidecladding 22600BCd may also be made up of the bottom interspaceddielectric current conduction layer BIDC 21350 with refractive indexn_(BIDC) 21350 n.

In as yet another embodiment, part of a bottom electro-active waveguidecladding 22600BCd may also be made up of the bottom side conduction andOhmic contact layer BSCOC 21300 with refractive index n_(BSCOC) 21300 n.

In as yet another embodiment, part of a bottom electro-active waveguidecladding 22600BCd may also be made up of the bottom side conduction andOhmic contact layer ISTRBCd 21200B with refractive index n_(ISTRBCd)21200Bn.

As to which layer shall be considered as the waveguide cladding is thatin the waveguide cladding material region (a material region thatsurround the waveguide core), the energy density of the guided modeshall decay largely exponentially in a direction away from the waveguidecore, as is known to those skilled in the art. This “waveguide claddingregion” may be made of a single layer or spot (i.e. small cluster) ofmaterial or a collection of multiple layers or spots (i.e. smallclusters) of connected materials (including air as a material). A spotis a three-dimensional cluster of material volume. The waveguidecladding refractive index n_(Cd) (e.g. as use in the in the nextsection) shall be taken as the averaged refractive index of thiscollection of multiple layers/spots of cladding materials that can haveone layer/spot or plurality of layer/spots. The waveguide core shall betaken as the material region close to the center energy portion of theoptical beam in which the material refractive index n_(MAT) is largerthan or equal to the beam propagating refractive index n_(BEC) and thewaveguide core refractive index n_(Co) (e.g. as use in the in the nextsection) shall be taken as the averaged refractive index of the entirecore material region (which again can be composed of layers or spots ofmaterials). For the purpose of illustration and not limitation, it isuseful to divide the cladding regions to be the top waveguide claddingsituated above the waveguide core, the bottom waveguide claddingsituated below the waveguide core, the left waveguide cladding situatedto the left of the waveguide core, and the right waveguide claddingsituated to the right of the waveguide core.

High Refractive Index Contrast and Mode-Medium Overlap

For the purpose of definition, it is useful to define a refractive indexcontrast parameter as described below. If a waveguide core refractiveindex is n_(Co) and the waveguide cladding (as defined by theexponential energy decay above) immediately adjacent to the waveguidecore has a refractive index n_(Cd), then we can define a waveguidecore-to-cladding refractive index difference square to be n_(rd)²=(n_(Co) ²−n_(Cd) ²) and a refractive index contrast ratio to be:R_(cts)=n_(rd) ²/(n_(Co) ²+n_(Cd) ²). For the purpose of definition andnot limitation, we define very-strongly wave guiding regime to be whenR_(cts)>0.5 or R_(cts)=0.5. It is also useful to define themedium-strongly wave guiding regime to be when 0.5>R_(cts)>0.2 orR_(cts)=0.2, weakly guiding regime to be when 0.2>R_(cts)>0.02 orR_(cts)=0.02 and the very-weakly guiding regime to be when 0.02>R_(cts).

In a preferred embodiment, the electro-active waveguiding core structureEWCoS 22600 is in the very-strongly guiding or medium-strongly guidingregime at least in the vertical direction (direction perpendicular tothe substrate) in which the refractive index contrast of the waveguidecore layer with the top and bottom cladding immediately adjacent to theelectro-active waveguide core given by: R_(cts)=(n_(Co) ²−n_(Cd)²)/(n_(Co) ²+n_(Cd) ²) is larger than or equal to about 0.2 or is largerthan or equal to about 0.5, where n_(Cd) is the refractive index of thetop or bottom cladding. In the case of waveguiding core structure EWCoS22600, n_(Co)=n_(WCo) where n_(WCo) 22600Con is the averaged refractiveindex of the central waveguide core layer WCo 22600Co, and n_(Cd) iseither n_(BIM), n_(BIDC), n_(laISTR) or n_(ISTRBCd) depending on whichone(s) is(are) the bottom cladding(s).

This strong waveguiding in the vertical direction will enable muchhigher mode confinement that will push higher fraction of the beamenergy into the electro-active layer. As a result, the phase shift inthe guided beam BEC 21140 given by a change in n_(BEC) 21140 n under anapplied voltage will be larger. This will result in substantially lowerswitching voltage. In order to increase this mode-medium overlappingfactor, it is useful to reduce the total thickness of the electro-activewaveguide core.

More precisely, it is useful to define the thickness d_(CORE) of theelectro-active waveguide core as the distance between a first topboundary and a first bottom boundary. The first top boundary is theboundary between the waveguide core and the top cladding immediatelyadjacent to the waveguide core, and the first bottom boundary is theboundary between the waveguide core and the bottom cladding immediatelyadjacent to the waveguide core. If d_(CORE) is smaller than(λ_(bm)(2*n_(Co))), the waveguide core is said to be in ultra-thinregime. If d_(CORE) is smaller than or equal to (λ_(bm)/n_(Co)) andlarger (λ_(bm)/(2*n_(Co))), then the waveguide core is said to be invery-thin regime. If d_(CORE) is smaller than or equal to(1.5*λ_(bm)/n_(Co)) and larger than (λ_(bm)/n_(Co)), the waveguide coreis said to be in medium-thin regime. If d_(CORE) is smaller than(3*λ_(bm)/n_(Co)) and larger than (1.5*λ_(bm)/n_(Co)), the waveguidecore is said to be in the thin regime. If d_(CORE) is larger than(3*λ_(bm)/n_(Co)), the waveguide core is said to be in the thick regime.

In a preferred embodiment for the modulator of the present invention, inorder to achieve additional enhanced performances such as very lowmodulation voltage, the electro-active waveguiding core structure EWCoS22600 shall be in the very-strongly guiding regime, and d_(CORE) shalleither be in the ultra-thin regime or very-thin regime.

In a preferred embodiment for the modulator of the present invention, inorder to achieve additional enhanced performances such as low modulationvoltage, the electro-active waveguiding core structure EWCoS 22600 shallbe in the medium-strongly guiding or very-strongly guiding regime, andd_(CORE) shall either be in the ultra-thin regime, very-thin regime, ormedium-thin regime.

In another preferred embodiment for the modulator of the presentinvention, the electro-active waveguiding core structure EWCoS 22600shall be in the weakly guiding regime, and d_(CORE) shall either be inthe ultra-thin, very-thin, medium-thin, or thin regime.

For example, if λ_(bm)=1550 nm, n_(EC)=3.0, then if d_(CORE) is smallerthan or equal to (λ_(bm)/n_(Co))=517 nm, it is in the very-thin regime,and if n_(Cd)=1.5, it also has R_(cts)>0.5 and hence is in thevery-strongly wave guiding regime as well, which will satisfy theco-requirements. Both requirements have to be satisfied in order to drawan exemplary benefit of the present invention such as to enhance the lowvoltage performance of the modulator.

It is useful to define the electro-active field overlapping factor moreprecisely. Let the electric field distribution of the guided mode ofoptical beam BEC 21140 be given by E_(OPT)(x,y) 21140E, where E_(OPT) isthe electric field strength, and x and y are the coordinates in thecross-sectional area of the beam. The mode m is typically thefundamental guided mode with a single intensity peak at the beam centerregion. Let Δn 21140 dn be the change in the optical propagatingrefractive index experienced by the beam under an applied electric fieldE_(EC)(x,y) 21500E (for the case of constant field E_(EC)(x,y)˜V_(EC)/D_(EC), where V_(EC) 21500VEC is the applied voltage and D_(EC)21500DEC is the effective physical distance for which the voltage V_(EC)is applied across) that again has a value profile depending on the x-ycross-sectional coordinates. For the case of refractive indexmodulation, then the phase shift Δφ 21140Ph experienced by an opticalbeam propagating through the modulator under an applied voltage V(zo,t)21500Vzo, where zo is the propagating distance along the modulation, isgiven by:

${{\Delta\varnothing} = {\left( {2{\pi/\lambda_{0}}} \right)\Delta \; n}},{{\Delta \; n} = {\int_{0}^{L}{\Gamma \; {V\left( {{zo},t} \right)}\ {{zo}}}}},{\Gamma = {\frac{1}{V_{EO}}\frac{\int{\int{\left( {1/2} \right)n_{eff}^{3}{r_{EO}\left( {x,y} \right)}{E_{EO}\left( {x,y} \right)}{{E_{OPT}\left( {x,y} \right)}}^{2}\ {x}\ {y}}}}{\int{\int{{{E_{OPT}\left( {x,y} \right)}}^{2}\ {x}\ {y}}}}}},$

where V_(EC) is the applied RF voltage that gives rise to E_(EC)(x,y),r_(EO)(x,y), 21500 rEO is an effective electro-optic coefficientdescribing how much the material's refractive index is changed under anapplied field, n_(eff) is the effective refractive index of thepropagating optical mode (same as n_(BEC)). The quantity Γ 21500ROMOF isthus called the RF-field, optical mode, and active medium overlappingfactor (also simply called the mode-medium overlapping factor). It isindependent on V_(EC) as E_(EC)(x,y) is proportional to V_(EC). Thevoltage V(z,t) is the actual applied voltage that may change withpropagation distance z and time t.

The voltage to the entire modulator V_(MOD) is approximately given byV_(EC) assuming the voltage drop between the electro-active layers andthe top or bottom electrode is small compared to V_(EC).

While the above mode-medium overlapping factor is illustrated for thecase of an electro-optic (EO) modulator, there are various otherdefinitions of mode-medium overlapping factor more suitable for otherapplications such as for the case of electro-absorption (EA) modulatoror the case involving optical absorption and gain medium, as is wellknown skilled in the art. These other definitions of mode-mediumoverlapping factor shall be used when appropriate and the specificdefinition of mode-medium overlapping factor is not meant to limit thepresent invention.

Low-Refractive-Index Ohmic-Transparent-Conductor & Metal Contact Case

The difficulty of obtaining high mode-medium overlapping or tight modeconfinement is that the optical mode will inevitably touch the metalcontact pad if the top vertical/side conduction and Ohmic contact layerTVSCOC is the usual doped semiconductor. One way to solve this problemin the present embodiment is to utilize Low-Refractive-Index OhmicTransparent Conductor (LRI-OTC) that is electrically conductive buthaving a low refractive index (the low-refractive-index criterion isdefined in the section on “High-refractive-index-contrast mode confiningstructure in electro-active region”). In that case, the mode can bestrongly confined in the electro-active waveguide core structure EWCS22600 region and will rapidly decay in the top electro-active waveguidecladding 22600UC. Layer 22600UC is basically the top vertical/sideconduction and Ohmic contact layer TVSCOC 21700. This would be possibleonly if n_(TVSC) is small compared to the refractive index n_(WCo)22600Con of the central waveguide core layer WCo 22600Co. However, it isimportant that layer TVSCOC 21700, now made of transparent conductor,shall have Ohmic-like contact with the top vertical conduction layer TVC21600 that may be semiconductor.

As an embodiment, layer TVSCOC 21700 is a low-refractive-indextransparent conductor with Ohmic contact capability with layer TVC21600. In that case, the material for layer TVSCOC 21700 will be called“Low-Refractive-Index Ohmic Transparent Conductor” (LRI-OTC). Ohmictransparent conductor differs from just transparent conductor as theyhave to have “low electrical contact resistance” with the nextconduction layer in contact with it to pass current down to theelectro-active layer without causing high voltage across the contactsurface. For the purpose of illustration and not limitation, the nextconduction layer is typically N-doped or P-doped semiconductor.Preferably, it shall also have low “low electrical contact resistance”with appropriate metal electrode. Materials for LRI-OTC include but arenot limited to transparent conducting oxide (TCO) materials such asIn₂O₃ (or various Indium Oxides), ZnO (Zinc Oxides), ITO (Indium TinOxides), GITO (Gallium Indium Tin Oxides), Gallium Indium Oxide (GIO),ZITO (Zinc Indium Tin Oxides), CdO (Cadmium Oxides), or materialscontaining any one or more than one of these oxides.

In as yet another embodiment, layer TVSCOC 21700 can also be alow-refractive-index transparent conductor. In that case, the materialfor layer TVSCOC 21700 will be called “Low-Refractive-Index TransparentConductor” (LRI-TC). Transparent conductor (TC) differs from Ohmictransparent conductor (OTC) as they do not need to have “low electricalcontact resistance” with the next conduction layer in contact with it.For example, one can have modulators in which layer TVSCOC 21700directly applies the electric field to the active material withoutfurther conducting the voltage down to the next layer. In that case, theother layers such as layer TVC 21600 may be undoped or an intrinsicsemiconductor that does not conduct electric voltage or current.

In another embodiment, in order to achieve high frequency response, itis also desirable that the Ohmic contact between layer TVC 21600 andTVSCOC 21700 has low Ohmic contact resistance. Ohmic-like contact meansthe relation between the voltage and the current is largely linear. Forthe purpose of illustration and not limitation, low Ohmic contactresistance between any two materials A and B generally means thevoltage-over-current ratio for a current going between material A andmaterial B is not substantially worse than the total sum of otherelectrical resistances that will affect the frequency response of themodulator.

In one exemplary embodiment, on top of layer TVSCOC 21700 with LRI-OTCmaterial is metal pad that gives good metal Ohmic contact with theLRI-OTC material used. In an embodiment, the metal pad is the first topmiddle metal contact pad FTMM 21800M with thickness d_(FTMM) 21800Md,width w_(FTMM) 21800Mw, and length g_(FTMM) 21800Mg.

This case is referred to as “top LRI-OTC-metal contact case”.

Side-Conduction and Metal Contact Case

Other alternative contacts include having layer TVSCOC 21700 to extendside way and have metal Ohmic contact on the side away from the centerregion of layer TVSCOC 21700 as shown in FIG. 23. In that case, Ohmiccontact can be achieved from the metal to layer TVSCOC 21700 and theoptical mode will not touch the metal, which can keep the optical modeto be low loss. This is referred to as “top side-conduction-metalcontact case”.

Still another alternative structure involved having the “top lateralconduction geometry with metal contact” but also a top lowlossdielectric material region TDMR 21810 as shown in FIG. 24. Thisdielectric material can be chosen to have low refractive index and henceacting as a top electro-active waveguide cladding. Alternatively, themetal can even go on top of TDMR 21810 to make this top lateralconduction structure mechanically robust as shown in FIG. 25. Thus,there are various ways to realize what we refer to as the “top lateralconduction geometry with metal contact”.

Still another alternative structure involved having the “bottom LRI-OTC”in that the bottom interspaced material layer BIM 21250 between bottomside-conduction and Ohmic-contact layer BSCOC 21300, and ISTR 21200,with thickness d_(BIM) 21250 d is made of LRI-OTC. This enables athicker layer for conducting current and voltage from the bottomelectrode(s) with electrode(s) either at the bottom of the LRI-TOC layeror on top of the LRI-TOC layer (e.g. using top via hole on one or bothsides of the ALS to contact the LRI-TOC layer through layer 21300) ordirectly on top of the bottom side-conduction and Ohmic contact layerBSCOC 21300.

Lateral Optical Mode Confinement

Note that in another embodiment, the width w_(TVSC) 21700 w of layer21700 can act on the optical mode of guided beam BEC 21140 laterally soas to confine the optical mode. Such lateral mode confinement is called“rib waveguide” structure, which is known to have low optical loss.Thus, in an embodiment, layer 21700 also forms a rib-waveguidestructure.

It must be understood that there are various ways to confine the opticalmode laterally, including a small lateral width w_(EC) 21500 w for asmall vertical portion of the electro-active layer 21500, which can alsoconfine the optical mode laterally, called “lateral mode confinement”structure. Thus, in another embodiment, the electro-active layer is a“lateral mode confinement” structure.

Similarly, a small lateral width w_(SVC) 21600 w or w _(FVC) 21400 w fora small vertical portion of layer 21600 or 21400 can also confine theoptical mode laterally. Thus in another embodiment, the bottom verticalconduction layer BVC 21400 is a “lateral mode confinement” structure. Inas yet another embodiment, the top vertical conduction layer TVC 21600is a “lateral mode confinement” structure.

In as yet another embodiment, the bottom interspaced dielectric currentconduction layer BIDC 21350 with thickness d_(BIDC) 21350 d, layer widthw_(BIDC) 21350 w, and an averaged refractive index n_(BIDC) 21350 n is a“lateral mode confinement” structure.

In as yet another embodiment, the bottom interspaced dielectric currentconduction layer BIM 21250 with thickness d_(BIM) 21250 d, layer widthwas 21250 w, and an averaged refractive index n_(BIM) 21250 n is a“lateral mode confinement” structure.

In as yet another embodiment, the input supporting structure ISTR 21200with thickness d_(ISTR) 21200 d, layer width w_(ISTR) 21200 w, and arefractive index n_(ISTR) 21200 n is a “lateral mode confinement”structure.

In as yet another embodiment, the output supporting structure OSTR 29200with thickness d_(OSTR) 29200 d, layer width w_(OSTR) 29200 w, and arefractive index n_(OSTR) 29200 n is a “lateral mode confinement”structure.

In as yet another embodiment, the top lowloss dielectric material regionTDMR 21810 with thickness d_(TDMR) 21810 d, layer width w_(TDMR) 21810w, and an averaged refractive index n_(TDMR) 21810 n is a “lateral modeconfinement” structure.

In as yet another embodiment, the top interspaced dielectric conductionlayer TIDC 21650 with thickness d_(TIDC) 21650 d, layer width w_(TUDC)21650 w, and an averaged refractive index n_(TIDC) 21650 n is a “lateralmode confinement” structure.

Reducing Modulator Junction Capacitance

In a preferred embodiment, the small lateral width w_(EC) 21500W for asmall vertical portion of the electro-active layer EC 21500 acts as a“lateral mode confinement” structure, but at the same time also reducesthe capacitance between the top and bottom electric-field applyingjunction in the modulator structure. This is because capacitance isproportional to the plate area and the lateral width w_(EC) 21500W willdefine the effective capacitance plate area across the PN (or PqN) layerin layer 21500, with P side serving as one capacitance plate and N sideserving as another capacitance plate, spaced by the carrier depletionwidth between the P and N doped material regions, as is known to thoseskilled in the art. Reducing the modulator junction capacitance canincrease the modulator frequency response. This will be referred to ascapacitance reduction via EC-layer width reduction. This can also beapplied to either layer 21400 or layer 21600 if a PN junctionresponsible for part of the total device capacitance is in layer 21400or 21600. In that case, the width of either layer 21400 or 21600 or bothshall be carefully chosen to reduce the total device capacitance.

Separate Lateral Mode Confinement and Modulator Junction CapacitanceReduction

Note that it is possible to implement this “capacitance reduction viaEC-layer width reduction” and still use the narrowed width of otherlayers to confine the optical mode laterally if the narrowed width ofother layers is comparable to or smaller than this EC-layer width. Thismay have certain advantage by having low capacitance but also by usingthe top vertical/side conduction and Ohmic contact layer TVSCOC 21700**(or any other layer between this layer and the layer ISTR 21200including layer ISTR 21200 itself, except the EC-layer) to confine themode laterally. This case is shown in FIG. 26. (**such as when layer21700 is an Ohmic Transparent Conductor case; for side metal contactcase, it will be difficult to use layer 21700 to confine the opticalbeam mode. In that case, one can alternatively use the top interspaceddielectric conduction layer TIDC 21650 to confine the optical modelaterally)

The Use of Highly Doped Quantum Wells for Lower Modulation Voltage

This structure enables the modulation voltage to be drastically reducedusing high carrier doping. While both N and P doping can be used, forthe purpose of illustration and not limitation, the preferred embodimentis the use of N-doping in the active electro-active region as P dopingwill cause higher optical absorption loss than N doping at the samedopant density. The higher the doping density, the smaller the carrierdepletion width at the PN junction and the larger the PN-junctioncapacitance. For a conventional modulator, the doping density is limitedto N=10¹⁷/cm³ as otherwise the high junction capacitance will begin toseverely limit the frequency bandwidth of the modulator. In theapplications below, the doping is made into the EC layer that may or maynot have quantum wells present. The presence of quantum wells mayenhance the refractive index change due to change in carrierband-filling in the quantum wells under an applied voltage. However, theabsence of quantum wells will also work in that refractive index willalso be changed due to change in carrier band-filling under an appliedvoltage. Thus, when quantum wells are mentioned, it is for the purposeof illustration and not limitation. The presence of quantum wells alsoenables refractive index change due to quantum confined Stark effects asnoted above, which can further increase the change in the refractiveindex under an applied voltage. The quantum wells can be strained,unstrained, double-well, or multi-well quantum wells as is known tothose skilled in the art.

In the present invention, in one application area, the EC layer hasregion with high-level doped carrier density with P-type or N-typedoping and a doping density at or higher than 2×10¹⁷/cm³ and lower than5×10¹⁷/cm³ primarily but not exclusively for low modulation voltageV_(MOD) 20000V or low modulation RF power P_(MOD) 20000P, and low-losshigh-frequency modulator applications. In an exemplary embodiment, forillustration and not limitation, low means V_(MOD)<2 Volt or P_(MOD)<80mW. Typically V_(MOD) and P_(MOD) are approximately related by theR_(LOAD) 20000R transmission line resistance or load resistance:P_(MOD)=V_(MOD) ²/R_(LOAD). In an exemplary embodiment, R_(LOAD)=50Ohms. This is referred to as having the quantum wells in highly-dopedregime.

In another application area, the EC layer has region withmedium-high-level doped carrier density with P-type or N-type doping anda doping density at or higher than 5×10¹⁷/cm³ and lower than1.5×10¹⁸/cm³ primarily but not exclusively for medium-low modulationvoltage V_(MOD) or medium-low modulation RF power P_(MOD), and low-losshigh-frequency modulator applications.

In an exemplary embodiment, for illustration and not limitation,medium-low means V_(MOD)<1 Volt or P_(MOD)<20 mW. This is referred to ashaving the quantum wells in medium-highly-doped regime.

In as yet another application area, the EC layer has region withvery-high-level doped carrier density with P-type or N-type doping and adoping density at or higher than 1.5×10¹⁸/cm³ and lower than 5×10¹⁸/cm³primarily but not exclusively for very-low modulation voltage V_(MOD) orvery-low modulation RF power P_(MOD), and low-loss high-frequencymodulator applications. In an exemplary embodiment, for illustration andnot limitation, very-low means V_(MOD)<0.6 Volt or P_(RF)<7 mW. This isreferred to as having the quantum wells in very-highly-doped regime.

In as yet another application area, the EC layer has region withultra-high-level doped carrier density with P-type or N-type doping anda doping density at or higher than 5×10¹⁸/cm³ primarily but notexclusively for ultra-low modulation voltage V_(MOD) or ultra-lowmodulation RF power P_(MOD), and low-loss high-frequency modulatorapplications. In an exemplary embodiment, for illustration and notlimitation, ultra-low means V_(MOD)<0.2 Volt or P_(MOD)<0.8 mW. This isreferred to as having the quantum wells in ultra-highly-doped regime.

The quantum wells can be strained, unstrained, double-well quantumwells, or multi-well as is known to those skilled in the art.

While the preferred embodiments and advantages of the invention havebeen illustrated and described, it will be clear that the invention isnot limited to these embodiments and advantages only. Numerousmodifications, changes, variations, substitutions and equivalents willbe apparent to those skilled in the art without departing from thespirit and scope of the invention as described.

To further illustrate the present invention, for the purpose ofillustration and not limitation, we describe a few exemplary devicesbelow.

A First Exemplary Device of Electro-Optic Modulator with Side ConductionGeometry

A preferred embodiment of an exemplary device is Modulator Device 20000with the following specifications referred to as a first exemplarydevice of electro-optic modulator with side-conduction geometry:

Substrate SUB 21100 is silicon wafer substrate with a thickness of about0.3 mm. Input connecting waveguide gore ICWCo 22200 is made of siliconfor which its averaged material refractive index n_(ICWCo) 22200 n isaround n_(ICWCo)=3.6, thickness d_(ICWCo) 22200 d is d_(ICWCo)=250 nm,and width W_(ICWCo) 22200 w is W_(ICWCo)=400 nm.

Input connecting-waveguide bottom cladding material ICWBCd 22200B issilicon dioxide (SiO₂), for which its refractive Index n_(ICWBCd)22200Bn is n_(ICWBCd)=1.45.

Input connecting waveguide top cladding material ICWTCd 22200T issilicon dioxide (SiO₂) for which its refractive Index n_(ICWTCd) 22200Tnis 1.45.

Input connecting waveguide left cladding material ICWLCd 22200L issilicon dioxide (SiO₂), for which its refractive Index n_(ICWLCd)22200Ln is 1.45

Input connecting waveguide right cladding material ICWRCd 22200R issilicon dioxide (SiO₂), for which its refractive Index n_(ICWRCd)22200Rn is 1.45

The above form an input connecting waveguide ICWG 22200WG. Thecore-cladding refractive-index difference n_(Rd) defined by n_(Rd)²=(n_(Co) ²−n_(Cd) ²) for waveguide ICWG 22200WG is n_(Rd)²=(3.6²−1.45²)=10.86 with n_(Co)=3.6 and n_(Cd)=1.45. Its averagedCladding Refractive Index is given by n_(aICWCd)=(n_(ICWBCd)²×A_(ICWBCd)+n_(ICWTCd) ²×A_(ICWTCd)+n_(ICWRCd) ² A_(ICWRCd)+n_(ICWLCd)² A_(ICWLCd))/(A_(ICWBCd)+A_(ICWTCd)+A_(ICWRCd)+A_(ICWLCd))^(0.5)=1.45.Its averaged Core Refractive Index is given by n_(aCo)=(n_(Co1)²×A_(Co1)+n_(Co2) ²×A_(Co2)+n_(Co3) ² A_(Co3)+ . . . +n_(Com) ²A_(Com))/(A_(Co1)+A_(Co2)+A_(Co3)+ . . . +A_(Com))^(0.5)=3.6.

The input optical beam IBM 22140 has propagating refractive indexn_(IBM) 22140 n, for which nm is approximately 2.8 with optical powerP_(bm) 22140P approximately 1 mW, electric field polarization E_(bm)22140E to be in the horizontal direction parallel to the substratesurface. It has a beam effective area A_(bm) 22140A of A_(bm)=0.04 μm²and an optical wavelength centered at λ_(bm) 22140L with λ_(bm)=1550 nmwith plurality of (N) frequency channels λ_(bm1)=1548 nm, λ_(bm2)=1549nm, λ_(bm3)=1550 nm, λ_(bm4)=1551 nm, and λ_(bm3)=1552 nm centered atλ_(bm)=1550 nm.

Input Beam Coupler Structure (IBCS) Region

The input tapering waveguide core ITWCo 223000 is made of silicon. Itswidth at a location z1. ITWCo-z1 22300 z 1 is denoted as widthw_(ITWCo-z1) 22300 w-z 1. This width is tapered from width at z1=0w_(ITWCo-z1=0) 22300 w-z 1=0 that has a value of w_(ITWCo-z1=0)=400nanometers (nm) to a width at z1>0 w_(ITWCo-z1>0) 22300 w-z 1>0 that isnarrower than 400 nm in a linear fashion.

The thickness of the tapering waveguide core d_(ITWCo-z1) 22300 d-z 1made of silicon is d_(ITWCo-z1)=250 nm with a refractive indexn_(ITWCo-z1) 22300 n-z that is n_(ITWCo-z1)=3.6.

The total length of tapering waveguide g_(ITWCo) 22300 g is g_(ITWCo)=20micrometers (μm). The width of the waveguide core at the end of thetapering at z1=g_(ITWCo) is w_(ITWCo-g) 22300 w-g with w_(ITWCo-g)=50nm.

Input supporting structure ISTR 21200 has width w_(ISTR) 21200 w withw_(ISTR)=50 nm and thickness d_(ISTR) 21200 d with d_(ISTR)=250 nm andlength g_(ISTR) 21200 g with g_(ISTR)=20 micrometers. It has aneffective layer averaged refractive index n_(laISTR) 21200 nla withn_(laISTR)<2.5.

Left cladding material ISTRLCd 21200L is air and has a refractive indexn_(ISTRLCd) 21200Ln given by n_(ISTRLCd)=1, and Right cladding materialISTRRCd 21200R is air and has a refractive index n_(ISTRRCd) 21200Rngiven by n_(ISTRRCd)=1. Its bottom cladding ISTRBCd 21200B is silicondioxide (this is part of a Burried-Oxide BOX layer in a typicalSilicon-On-Insulator SOI wafer) with averaged refractive indexn_(ISTRBCd) 21200Bn of n_(ISTRBCd)=1.45.

The top cladding ITWTCd-z1 22300T-z1 before going into the ALS region issilicon dioxide (SiO₂) has refractive index n_(ITWTCd-z1) 22300Tn-z1with n_(ITWTCd-z1)=1.45.

The bottom cladding ITWBCd-z1 22300B-z1 before going into the ALS regionis silicon dioxide (SiO₂) has refractive index n_(ITWTCd-z1) 22300Bn-z1with n_(ITWTCd-z1)=1.45.

The left cladding ITWLCd-z1 22300L-z1 before going into the ALS regionis silicon dioxide (SiO₂) has refractive index n_(ITWBCd-z1) 22300Ln-z1with n_(ITWBCd-z1)=1.45.

The right cladding ITWRCd-z1 22300R-z1 before going into the ALS regionis silicon dioxide (SiO₂) has refractive index n_(ITWRCd-z1) 22300Rn-z1with n_(ITWRCd-z1)=1.45.

In this exemplary embodiment,n_(ITWTCd-z1)=n_(ITWBCd-z1)=n_(ITWLCd-z1)=n_(ITWRCd-z1)=n_(ITWTCd), andn_(ICWTCd)=n_(ICWBCd)=n_(ICWLCd)=n_(ICWRCd). Input tapering waveguidecore ITWCo 22300 starting at z1=z1ALS 22300 z 1ALS, where z1ALS=10micrometers, is laid with an active layer structure ALS 22500.0<z1ALS<g_(ITWCo).

Active Layer Structure-Beam Transport into the Structure

The active layer structure ALS 22500 is shown by the Table 3-1 below:

TABLE 3-1 ALS 22500 Layer Doping/ Layer Number Thickness NPNN TCO CASE(1/cm³) BIM 100 nm  In₂O₃ (21250) BSCOC 1 100 nm  InGaAsP 1.3 um N = 1 ×(21300) (Bottom layer-just 10¹⁹ above the substrate) BIDC 2 40 nm InP N= 1 × (21350LN) 10¹⁹ BVC 3 20 nm InGaAsP 1.3 um N = 1 × (21400) 10¹⁹ EC4 10 nm AlGaInAs 1.3 um N₁ = 1 × (21500LN₁) 10¹⁹ EC 5 4 nm barrierAlGaInAs/1.1 um/−0.8% MN₁ = 4 × (21500MLN₁) tensile strained 10¹⁷ EC 6 2× 7 nm AlGaInAs/1.1 um/−0.8% MN₂ = 4 × (21500MLN₂) barrier insidetensile strained 10¹⁷ EC 7 3 × 6.5 nm AlGaInAs/1.55 um/0.9% MN₃ = 4 ×(21500MLN₃) Well (PL = compressive strained 10¹⁷ 1350 nm) EC 8 4 nmbarrier AlGaInAs/1.1 um/−0.8% MN₄ = 4 × (21500MLN₄) tensile strained10¹⁷ EC 9 43 nm AlGaInAs 1.3 um MN₅ = 4 × (21500MLN₅) 10¹⁷ EC 10 20 nmAlGaInAs 1.3 um P₁ = 1 × (21500LP₁) 10¹⁸ TVC 11 25 nm InGaAsP 1.3 um P₂= 1 × (21600P₂) 10¹⁸ TVC 12 20 nm InGaAsP 1.3 um N₂ = 1 × (21600N₂) 10¹⁹TIDC 13 20 nm InP N = 1 × (21650) 10¹⁹ TVSCOC 14 40 nm InGaAsP (Toplayer) N = 1 × (21700) 10¹⁹ Total 380 nm 

In the table, the materials are unstrained (with InP as the substrate)if not specified as strained. The wavelength specified will be thematerial bandgap wavelength of the quaternary material involved (properchoice of the material composition is needed to achieve the requiredmaterial bandgap and strain when grown on InP substrate).

Bottom Side Conduction and Ohmic Contact Layer

The active layer structure ALS 22500 has a bottom side conduction andOhmic contact layer BSCOC 21300 that is InGaAsP layer given by layer 1in Table 3-1 with thickness d_(BSC) 21300 d, where d_(BSC)=100 nm andwidth w_(BSC) 21300 w, where w_(BSC) is approximately 54 micrometersalong most of the length of the ALS. Its refractive index n_(BSC) 21300n is n_(BSC)=3.4.

Bottom Interspaced Material Layer

The bottom interspaced material layer BIM 21250 is made of aLow-Refractive-Index Ohmic Transparent Conducting (LRI-OTC) materialcomposed of Indium oxide (In₂O₃) with thickness d_(BIM) 21250 d equalsto d_(BIM)=100 nm, width w_(BIM) 21250 w equals to w_(BIM)=54micrometers, and average refractive index n_(BIM) 21250 n equals ton_(BIM)=1.7.

Bottom Metal Contact Pads

The first bottom left metal contact pad FBLM 21900L is a multi-layermetal made up of (17 nm Au followed by 17 nm Ge followed by 17 nm Aufollowed by 17 nm Ni followed by 1000 nm Au) deposited on top of the topsurface of n-doped layer 21300 given by layer 1 in Table 3-1. The totalthickness of the metal contact pad is d_(FBLM) 21900Ld, withd_(FBLM)=1068 nm, and width w_(FBLM) 21900Lw, where w_(FBLM) isapproximately 20 micrometers. The length of the metal contact padg_(FBLM) 21900Lg is approximately 500 micrometers.

The first bottom right metal contact pad FBRM 21900R is multilayer metalmade up of (17 nm Au followed by 17 nm Ge followed by 17 nm Au followedby 17 nm Ni followed by 1000 nm Au) deposited on top of the top surfaceof n-doped layer 21300 given by layer 1 in Table 3-1. The totalthickness of the metal contact pad is d_(FBRM) 21900Rd, withd_(FBRM)=1068 nm, and width w_(FBRM) 21900Rw, where w_(FBRM) isapproximately 20 micrometers. The length of the metal contact padg_(FBRM) 21900Rg is approximately 500 micrometers.

Bottom Metal Electrodes

On top of the first bottom left metal contact pad FBLM 21900L isdeposited the first bottom left metal electrode FBLME 21120L which isgold of thickness of approximately 2 micrometer thick.

On top of the first bottom right metal contact pad FBRM 21900R isdeposited the first bottom right metal electrode FBRME 21120R which isgold of thickness of approximately 2 micrometer thick.

Bottom Interspaced Dielectric Current Conduction Layer

Bottom interspaced dielectric current conduction layer BIDC 21350 is an-doped InP given by layer 2 in Table 3-1 with thickness d_(BIDC) 21350d equals to d_(BIDC)=40 nm, width w_(BIDC) 21350 w equals to w_(BIDC)=54micrometers, and average refractive index n_(BIDC) 21350 n equals toabout n_(BIDC)=3.0.

Bottom Vertical Current Conduction Layer

Bottom vertical current conduction layer BVC 21400 is n-doped InGaAsPgiven by layer 3 in Table 3-1 with thickness d_(BVC) 21400 d equals tod_(BVC)=20 nm, width w_(BVC) 21400 w equals to w_(BVC)=2 micrometers,and an averaged refractive index n_(BVC) 21400 n equals to n_(BVC)=3.4.

Electro-Active Layer

Electro-active layer EC 21500 is made up of layers 4, 5, 6, 7, 8, 9, 10in Table 3-1 with an averaged refractive index of the entire layer givenby n_(EC) 21500 n with n_(EC) equals to approximately n_(EC)=3.4. Underan applied electric field, there will be a change in averaged refractiveindex dn_(EC) 21500 dn. The average refractive index becomesn_(EC)(new)=n_(EC)+dn_(EC).

The total thickness d_(EC) 21500 d of this Electro-active layer isd_(EC)=114.5 nm. Its width w_(EC) 21500 w is equal to w_(EC)=2micrometers.

The electro-active layer has a PqN junction at layer 4 to 10 for whichlayer 4 is layer 21500LN₁ that is N-doped with a dopant density of21500N₁=1×10¹⁹/cm³ and layer 10 is layer 21500LP₁ that is P-doped with adopant density of 21500P₁=1×10¹⁸/cm³

The intermediate layers 21500MLN_(m) are all N-doped with a dopantdensity of 21500MN_(m)=4×10¹⁷/cm³.

The applied field E_(EC) 21500E (which may cause a current C_(EC) 21500Cto flow) is across the entire electro-active layer with a negativevoltage applied to the top and positive voltage applied to the bottom ofthis entire electro-active layer known to those skilled in the art asrevered bias (with respect to the PN junction in the electro-activelayer) of voltage V_(R) 21500VR so the applied electro-active V_(EC)21500VEC is V_(R).

The voltage applied to the electrodes of the modulator V_(MOD) 20000V isapproximately given by V_(EC).

Top Vertical Current Conduction Layer

Top vertical current conduction layer TVC 21600 is given by layer 11 and12 in Table 3-1 made up of InGaAsP layer that is composed of 25 nm-thicklayer 21600LP₂ that is P-doped with dopant density 21600P₂=1×10¹⁸/cm³,followed by 20 nm-thick N-doped InGaAsP layer 21600LN₂ with dopantdensity 21600N₂=1×10¹⁹/cm³. The total thickness for TVC 21600 is d_(TVC)21600 d with d_(TVC)=45 nm. Its width is W_(TVC) 21600 w equals toW_(TVC)=2 micrometers, and its averaged refractive index is n_(TVC)21600 n equals to n_(TVC)=3.4. This N₂P₂ junction forms a forward-BiasedPN Junction (or Tunnel PN Junction). It forms a PN-changing PN junction(called PNCPN junction) 21600PNCPN.

Top Interspaced Dielectric Current Conduction Layer

Top interspaced dielectric conduction layer TIDC 21650 is N-doped InPlayer given by layer 13 in Table 3-1 with thickness d_(TIDC) 21650 dequals to d_(TIDC)=20 nm, width w_(TIDC) 21650 w equals to w_(TIDC)=8micrometers, and averaged refractive index n_(TIDC) 21650 n equals ton_(TIDC)=3.0.

Top Vertical/Side Conduction and Ohmic Contact Layer

Top vertical/side conduction and Ohmic contact layer TVSCOC 21700 ismade up of InGaAsP given by layer 14 in Table 3-1 with thicknessd_(TVSC) 21700 d equals to d_(TVSC)=40 nm, width w_(TVSC) 21700 w equalsto w_(TVSC)=8 micrometers, and an averaged refractive index n_(TVSC)21700 n equals to n_(TVSC)=3.4.

Top Metal Contact Pads

The first top left metal contact pad FTLM 21800L is multilayer metalmade up of (17 nm Au followed by 17 nm Ge followed by 17 nm Au followedby 17 nm Ni followed by 1000 nm Au) deposited on top of the top surfaceof n-doped layer 21700 given by layer 14 in Table 3-1. The totalthickness of the metal contact pad is d_(FTLM) 21800Ld, withd_(FTLM)=1068 nm, and width W_(FTLM) 21800Lw, where w_(FTLM) isapproximately 3 micrometers. The length of the metal contact padg_(FTLM) 21800Lg is approximately 500 micrometers.

The first top right metal contact pad FTRM 21800R is multilayer metalmade up of (17 nm An followed by 17 nm Ge followed by 17 nm Au followedby 17 nm Ni followed by 1000 nm Au) deposited on top of the top surfaceof n-doped layer 21700 given by layer 14 in Table 3-1. The totalthickness of the metal contact pad is d_(FTRM) 21800Rd, withd_(FTRM)=1068 nm, and width w_(FTRM) 21800Rw, where w_(FTRM) isapproximately 3 micrometers. The length of the metal contact padg_(FTRM) 21800Rg is approximately 500 micrometers.

There is no top middle metal contact pad FTMM 21800M.

Top Metal Electrodes

On top of the first top left metal contact pad FTLM 21800L is depositedthe first top left metal electrode FTLME 21130L which is gold ofthickness of approximately 2 micrometer thick.

On top of the first top right metal contact pad FTRM 21800R is depositedthe first top right metal electrode FTRME 21130R which is gold ofthickness of approximately 2 micrometer thick.

Beam Transport to Electro-Active Waveguiding Core Structure

Input tapering waveguide region between z1=z1ALS 22300 z 1ALS andz1=g_(ITWCo) 22300 g, Tapering waveguide core width w_(ITWCo-z) 22300 wvaries down to a smaller value of w_(ITWCo-g)=50 nm at z1=g_(ITWCo)22300 g from its vale at z1=z1ALS 22300 z 1ALS. ClearlyW_(ITWCo-g)<<λ_(bm)/(2*n_(ITWCo)), with λ_(bm)=1550 nm andn_(ITWCo)=3.6, where * is number multiplication.

Output Connecting Waveguide

Output connecting waveguide core OCWCo 28200 has averaged RefractiveIndex n_(OCWCo)=n_(aOCWCo)=3.6, thickness d_(OCWCo) 28200 d isd_(OCWCo)=250 nm, and width W_(OCWCo) 28200 w is W_(OCWCo)=400 nm.

Output connecting waveguide OCWG 28200WG has Output connecting-waveguidebottom cladding material OCWBCd 28200B that is silicon dioxide (SiO₂)for which the refractive index n_(OCWBCd) 28200Bn is n_(OCWBCd)=1.45.

Output connecting waveguide top cladding material OCWTCd 28200T issilicon dioxide foe which the refractive index n_(OCWBCd) 28200Tn isn_(OCWTCd)=1.45.

Output connecting waveguide left cladding material OCWLCd 28200L issilicon dioxide for which the refractive index n_(OCWLCd) 28200Ln isn_(OCWLCd)=1.45.

Output connecting waveguide right cladding material OCWRCd 28200R issilicon dioxide for which the refractive index n_(OCWRCd) 28200Rn isn_(OCWRCd)=1.45.

The resulted averaged cladding refractive Index n_(aOCWCd) 28200 aCdn isn_(aOCWCd)=1.45.

Output Optical Beam OBM 28140

Output Beam Coupler Structure (OBCS) Region

Output tapering waveguide core OTWCo 28300 is made of silicon. Its widthat a location z2 OTWCo-z2 is denoted as width w_(OTWCo-z2) 28300 w-z 2.This width is tapered from width at z2=0 w_(OTWCo-z2=0) 28300 w-z 2=0that has a value of w_(OTWCo-z2=0)=400 nm to a width at z2>0w_(OTWCo-z2>0) 28300 w-z 2>0 that is narrower than 400 nm in a linearfashion.

The thickness of the tapering waveguide core d_(OTWCo-z2) 28300 d-z 2made of silicon is d_(OTWCo-z2)=250 nm with a refractive indexn_(OTWCo-z2) 28300 n-z 2 that is n_(OTWCo-z2)=3.6.

The total length of tapering waveguide g_(OTWCo) 28300 g is g_(OTWCo)=20micrometers (□m). The width of the waveguide core at the end of thetapering at z2=g_(OTWCo) is w_(OTWCo-g) 28300 w-g with w_(OTWCo-g)=50nm.

Output supporting structure OSTR 29200 has width w_(OSTR) 29200 w withw_(OSTR)=50 nm and thickness d_(OSTR) 29200 d with d_(OSTR)=250 nm andlength g_(OSTR) 29200 g with g_(OSTR)=20 micrometers. It has aneffective layer averaged refractive index n_(laOSTR) 29200 nla withn_(laOSTR)<2.5.

The top cladding OTWTCd-z2 28300T-z2 is silicon dioxide (SiO₂) hasrefractive index n_(OTWTCd-z2) 28300Tn-z2 with n_(OTWTCd-z2)=1.45 beforegoing into the ALS region.

The bottom cladding OTWBCd-z2 28300B-z2 is silicon dioxide (SiO₂) hasrefractive index n_(OTWBCd-z2) 28300Bn-z2 with n_(OTWBCd-z2) ²=1.45.

The left cladding OTWLCd-z2 28300L-z2 is silicon dioxide (SiO₂) hasrefractive index n_(OTWLCd-z2) 28300Ln-z2 with n_(OTWLCd-z2)=1.45.

The right cladding OTWRCd-z2 28300R-z2 is silicon dioxide (SiO₂) hasrefractive index n_(OTWRCd-z2) 28300Rn-z2 with n_(OTWRCd-z2)=1.45.

In this exemplary embodiment,n_(OTWTCd-z2)=O_(OTWBCd-z2)=n_(OTWLCd-z2)=n_(OTWRCd-z2)=n_(OCWTCd), andn_(OCWTCd)=n_(OCWBCd)=n_(OCWLCd)=n_(OCWRCd)

Output tapering waveguide core OTWCo 28300 starting at z2=z2ALS 28300 z2ALS, is laid with an active layer structure ALS 22500.0<z2ALS<g_(OTWCo).

Most of the output optical beam energy of beam OBM 28140 is transportedto output tapering waveguide core OTWCo 28300 from the electro-activewaveguiding core structure EWCoS 22600, through the output taperingwaveguide region between z2=z2ALS 28300 z 2ALS and z2=g_(OTWCo) 28300 g,where the output tapering waveguide core width w_(OTWCo-z2) 28300 w-z 2varies down to a smaller value of W_(OTWCo-g) at z2=g_(ITWCo) 28300 gfrom its vale at z2=z2ALS, 28300 z 2ALS. The tapering waveguide corewidth is reduced to well below half the optical wavelength in thewaveguide core given by λ_(bm)/(2×n_(OTWCo)) so thatw_(OTWCo-g)<<λ_(bm)/(2×n_(OTWCo)). After the energy transported from theelectro-active waveguiding core structure EWCoS 22600 that contains theelectro-active layer EC 21500 down to the output taper at z2=0 where thetaper core width is w_(OTWCo-z2=0) 28300 w 0 and w_(OTWCo-z2=0) 28200,the optical beam is denoted as output optical beam or beam OBM 28140.

Length of Active Layer Structure

The length of the active layer structure SL_(mod) 22550 is approximately500 micrometers.

High Refractive Index Contrast and Mode Overlapping

For the bottom cladding:Waveguide core refractive index is n_(co)=3.6Waveguide bottom cladding is n_(BCd)=1.45 (given by layer ISTRBC withn_(ISTRBCd)=1.45)Waveguide core-to-cladding refractive index difference square to ben_(rd) ²=(n_(co) ²−n_(BCd) ²)=10.86.Refractive index contrast ratio to be: R_(cts)=n_(rd) ²/(n_(co)²+n_(BCd) ²)=0.7, which is in the very-strongly guiding regime.For the top cladding:Waveguide core refractive index is n_(co)=3.6Waveguide bottom cladding is n_(TCd)=1 (given by material above TVSCOClayer which is air with n=1)Waveguide core-to-cladding refractive index difference square to ben_(rd) ²=(n_(co) ²−n_(TCd) ²)=11.96.Refractive index contrast ratio to be: R_(cts)=n_(rd) ²/(n_(co)²+n_(TCd) ²)=0.86, which is in the very-strongly guiding regime.A Second Exemplary Device of Electro-Absorption Modulator withTransparent Conductor Geometry

A preferred embodiment of an exemplary device is Modulator Device 20000with the following specifications referred to as a second exemplarydevice of electro-absorption modulator with Ohmic transparent conductorgeometry: The main difference between this and the First ExemplaryDevice is in Table 3-2, in which the active layer is designed forelectro-absorption modulation. Also, there are no left and right topmetal contact pads, only middle metal contact pad.

Substrate SUB 21100 is silicon wafer substrate with a thickness of about0.3 mm.

Input connecting waveguide gore ICWCo 22200 is made of silicon for whichits averaged material refractive index n_(ICWCo) 22200 n is aroundn_(ICWCo)=3.6, thickness d_(ICWCo) 22200 d is d_(ICWCo)=250 nm and widthW_(ICWCo) 22200 w is W_(ICWCo)=400 nm.

Input connecting-waveguide bottom cladding material ICWBCd 22200B issilicon dioxide (SiO₂), for which its refractive Index n_(ICWBCd)22200Bn is n_(ICWBCd)=1.45.

Input connecting waveguide top cladding material ICWTCd 22200T issilicon dioxide (SiO₂) for which its refractive Index n_(ICWTCd) 22200Tnis 1.45.

Input connecting waveguide left cladding material ICWLCd 22200L issilicon dioxide (SiO₂), for which its refractive Index n_(ICWLCd)22200Ln is 1.45

Input connecting waveguide right cladding material ICWRCd 22200R issilicon dioxide (SiO₂), for which its refractive Index n_(ICWRCd)22200Rn is 1.45

The above form an input connecting waveguide ICWG 22200WG. Thecore-cladding refractive-index difference n_(Rd) defined by n_(Rd)²=(n_(Co) ²−n_(Cd) ²) for waveguide ICWG 22200WG is n_(Rd)²=(3.6²−1.45²)=10.86 with n_(Co)=3.6 and n_(Cd)=1.45. Its averagedCladding Refractive Index is given by n_(aICWCd)=(n_(ICWBCd)²×A_(ICWBCd)+n_(ICWTCd) ²×A_(ICWTCd)+n_(ICWRCd) ² A_(ICWRCd)+n_(ICWLCd)² A_(ICWLCd))/(A_(ICWBCd)+A_(ICWTCd)+A_(ICWRCd)+A_(ICWLCd))^(0.5)=1.45.Its averaged Core Refractive Index is given by n_(aCo)=(n_(Co1)²×A_(Co1)+n_(Co2) ²×A_(Co2)+n_(Co3) ² A_(Co3)+ . . . +n_(Com) ²A_(Com))/(A_(Co1)+A_(Co2)+A_(Co3)+ . . . +A_(Com))^(0.5)=3.6.

The input optical beam IBM 22140 has propagating refractive indexn_(IBM) 22140 n, for which n_(IBM) is approximately 2.8 with opticalpower P_(bm) 22140P approximately 1 mW, electric field polarizationE_(bm) 22140E to be in the horizontal direction parallel to thesubstrate surface. It has a beam effective area A_(bm) 22140A ofA_(bm)=0.04 μm² and an optical wavelength centered at λ_(bm) 22140L withλ_(bm)=1550 nm with plurality of (N) frequency channels λ_(bm1)=1548 nm,λ_(bm2)=1549 nm, λ_(bm3)=1550 nm, λ_(bm4)=1551 nm, and λ_(bm3)=1552 nmcentered at λ_(bm)=1550 nm.

Input Beam Coupler Structure (IBCS) Region

The input tapering waveguide core ITWCo 223000 is made of silicon. Itswidth at a location z1. ITWCo-z1 22300 z 1 is denoted as widthw_(ITWCo-z1) 22300 w-z 1. This width is tapered from width at z1=0w_(ITWCo-z1=0) 22300 w-z 1=0 that has a value of w_(ITWCo-z1=0)=400nanometers (nm) to a width at z1>0 w_(ITWCo-z1>0) 22300 w-z 1>0 that isnarrower than 400 nm in a linear fashion.

The thickness of the tapering waveguide core d_(ITWCo-z1) 22300 d-z 1made of silicon is d_(ITWCo-z1)=250 nm with a refractive indexn_(ITWCo-z1) 22300 n-z that is n_(ITWCo-z1)=3.6.

The total length of tapering waveguide g_(ITWCo) 22300 g is g_(ITWCo)=20micrometers (μm). The width of the waveguide core at the end of thetapering at z1=g_(ITWCo) is w_(ITWCo-g) 22300 w-g with w_(ITWCo-g)=50nm.

Input supporting structure ISTR 21200 has width w_(ISTR) 21200 w withw_(ISTR)=50 nm and thickness d_(ISTR) 21200 d with d_(ISTR)=250 nm andlength g_(ISTR) 21200 g with g_(ISTR)=20 micrometers. It has aneffective layer averaged refractive index n_(laISTR) 21200 nla withn_(laISTR)<2.5.

Left cladding material ISTRLCd 21200L is air and has a refractive indexn_(ISTRLCd) 21200Ln given by n_(ISTRLCd)=1, and Right cladding materialISTRRCd 21200R is air and has a refractive index n_(ISTRRCd) 21200Rngiven by n_(ISTRRCd)=1. Its bottom cladding ISTRBCd 21200B is silicondioxide (this is part of a Burried-Oxide BOX layer in a typicalSilicon-On-Insulator SOI wafer) with averaged refractive indexn_(ISTRBCd) 21200Bn of n_(ISTRBCd)=1.45.

The top cladding ITWTCd-z1 22300T-z1 before going into the ALS region issilicon dioxide (SiO₂) has refractive index n_(ITWTCd-z1) 22300Tn-z1with n_(ITWTCd-z1)=1.45.

The bottom cladding ITWBCd-z1 22300B-z1 before going into the ALS regionis silicon dioxide (SiO₂) has refractive index n_(ITWBCd-z1) 22300Bn-z1with n_(ITWBCd-z1)=1.45.

The left cladding ITWLCd-z1 22300L-z1 before going into the ALS regionis silicon dioxide (SiO₂) has refractive index n_(ITWLCd-z1) 22300Ln-z1with n_(ITWLCd-z1)=1.45.

The right cladding ITWRCd-z1 22300R-z1 before going into the ALS regionis silicon dioxide (SiO₂) has refractive index n_(ITWRCd-z1) 22300Rn-z1with n_(ITWRCd-z1)=1.45.

In this exemplary embodiment,n_(ITWRCd-z1)=n_(ITWBCd-z1)=n_(ITWLCd-z1)=n_(ITWRCd-z1)=n_(ICWTCd), andn_(ICWTCd)=n_(ICWBCd)=n_(ICWLCd)=n_(ICWRCd). Input tapering waveguidecore ITWCo 22300 starting at z1=z1ALS 22300 z 1ALS, where z1ALS=10micrometers, is laid with an active layer structure ALS 22500.0<z1ALS<g_(ITWCo).

Active Layer Structure-Beam Transport into the Structure

The active layer structure ALS 22500 is shown by the Table 3-2 below:

TABLE 3-2 ALS 22500 Layer Doping/ Layer Number Thickness NPNN TCO CASE(1/cm³) BIM 100 nm  In₂O₃ (21250) BSCOC 1 100 nm  InGaAsP 1.3 um N = 1 ×(21300) (Bottom layer-just 10¹⁹ above the substrate) BIDC 2 40 nm InP N= 1 × (21350LN) 10¹⁹ BVC 3 20 nm InGaAsP 1.3 um N = 1 × (21400) 10¹⁹ EC4 10 nm AlGaInAs 1.3 um N₁ = 1 × (21500LN₁) 10¹⁹ EC 5 11 nm barrierAlGaInAs/1.19 um/−0.3% MI₁ (21500MLI₁) tensile strained EC 6 3 × 7 nmAlGaInAs/1.19 um/−0.3% MI₂ (21500MLI₂) barrier inside tensile strainedEC 7 4 × 11 nm AlGaInAs/1.55 um/0.31% MI₃ (21500MLI₃) Well (PL =compressive strained 1500 nm) EC 8 11 nm barrier AlGaInAs/1.19 um/−0.3%MI₄ (21500MLI₄) tensile strained EC 9 43 nm AlGaInAs 1.3 um MI₅(21500MLI₅) EC 10 20 nm AlGaInAs 1.3 um P₁ = 1 × (21500LP₁) 10¹⁸ TVC 1125 nm InGaAsP 1.3 um P₂ = 1 × (21600P₂) 10¹⁸ TVC 12 20 nm InGaAsP 1.3 umN₂ = 1 × (21600N₂) 10¹⁹ TIDC 13 20 nm InP N = 1 × (21650) 10¹⁹ TVSCOC 14240 nm  In₂O₃ (Top layer) (21700) Total 625 nm 

In the table, the materials are unstrained (with InP as the substrate)if not specified as strained. The wavelength specified will be thematerial bandgap wavelength of the quaternary material involved (properchoice of the material composition is needed to achieve the requiredmaterial bandgap and strain when grown on InP substrate).

Bottom Side Conduction and Ohmic Contact Layer

The active layer structure ALS 22500 has a bottom side conduction andOhmic contact layer BSCOC 21300 that is InGaAsP layer given by layer 1in Table 3-2 with thickness d_(BSC) 21300 d, where d_(BSC)=100 nm andwidth w_(BSC) 21300 w, where w_(BSC) is approximately 54 micrometersalong most of the length of the ALS. Its refractive index n_(BSC) 21300n is n_(BSC)=3.4.

Bottom Interspaced Material Layer

The bottom interspaced material layer BIM 21250 is made of aLow-Refractive-Index Ohmic Transparent Conducting (LRI-OTC) materialcomposed of Indium oxide (In₂O₃) with thickness d_(BIM) 21250 d equalsto d_(BIM)=100 nm, width w_(BIM) 21250 w equals to w_(BIM)=54micrometers, and average refractive index n_(BIM) 21250 n equals ton_(BIM)=1.7.

Bottom Metal Contact Pads

The first bottom left metal contact pad FBLM 21900L is a multi-layermetal made up of (17 nm Au followed by 17 nm Ge followed by 17 nm Aufollowed by 17 nm Ni followed by 1000 nm Au) deposited on top of the topsurface of n-doped layer 21300 given by layer 1 in Table 3-2. The totalthickness of the metal contact pad is d_(FBLM) 21900Ld, withd_(FBLM)=1068 nm, and width w_(FBLM) 21900Lw, where w_(FBLM) isapproximately 20 micrometers. The length of the metal contact padg_(FBLM) 21900Lg is approximately 500 micrometers.

The first bottom right metal contact pad FBRM 21900R is multilayer metalmade up of (17 nm Au followed by 17 nm Ge followed by 17 nm Au followedby 17 nm Ni followed by 1000 nm Au) deposited on top of the top surfaceof n-doped layer 21300 given by layer 1 in Table 3-2. The totalthickness of the metal contact pad is d_(FBRM) 21900Rd, withd_(FBRM)=1068 nm, and width w_(FBRM) 21900Rw, where w_(FBRM) isapproximately 20 micrometers. The length of the metal contact padg_(FBRM) 21900Rg is approximately 500 micrometers.

Bottom Metal Electrodes

On top of the first bottom left metal contact pad FBLM 21900L isdeposited the first bottom left metal electrode FBLME 21120L which isgold of thickness of approximately 2 micrometer thick.

On top of the first bottom right metal contact pad FBRM 21900R isdeposited the first bottom right metal electrode FBRME 21120R which isgold of thickness of approximately 2 micrometer thick.

Bottom Interspaced Dielectric Current Conduction Layer

Bottom interspaced dielectric current conduction layer BIDC 21350 is an-doped InP given by layer 2 in Table 3-2 with thickness d_(BIDC) 21350d equals to d_(BIDC)=40 nm, width w_(BIDC) 21350 w equals to w_(BIDC)=54micrometers, and average refractive index n_(BIDC) 21350 n equals toabout n_(BIDC)=3.0.

Bottom Vertical Current Conduction Layer

Bottom vertical current conduction layer BVC 21400 is n-doped InGaAsPgiven by layer 3 in Table 3-2 with thickness d_(BVC) 21400 d equals tod_(BVC)=20 nm, width w_(BVC) 21400 w equals to w_(BVC)=2 micrometers,and an averaged refractive index n_(BVC) 21400 n equals to n_(BVC)=3.4.

Electro-Active Layer

Electro-active layer EC 21500 is made up of layers 4, 5, 6, 7, 8, 9, 10in Table 3-1 with an averaged refractive index of the entire layer givenby n_(EC) 21500 n with n_(EC) equals to approximately n_(EC)=3.4. Underan applied electric field, there will be a change in averaged refractiveindex dn_(EC) 21500 dn. The average refractive index becomesn_(EC)(new)=n_(EC)+dn_(EC).

The total thickness d_(EC) 21500 d of this Electro-active layer isd_(EC)=160 nm. Its width w_(EC) 21500 w is equal to w_(EC)=2micrometers.

The electro-active layer has a PqN junction at layer 4 to 10 for whichlayer 4 is layer 21500LN₁ that is N-doped with a dopant density of21500N₁=1×10¹⁹/cm³ and layer 10 is layer 21500LP₁ that is P-doped with adopant density of 21500P₁=1×10¹⁸/cm³

The intermediate layers 21500MLN_(m) are all undoped (intrinsic).

The applied field E_(EC) 21500E (which may cause a current C_(EC) 21500Cto flow) is across the entire electro-active layer with a negativevoltage applied to the top and positive voltage applied to the bottom ofthis entire electro-active layer known to those skilled in the art asrevered bias (with respect to the PN junction in the electro-activelayer) of voltage V_(R) 21500VR so the applied electro-active V_(EC)21500VEC is V_(R).

The voltage applied to the electrodes of the modulator V_(MOD) 20000V isapproximately given by V_(EC).

Top Vertical Current Conduction Layer

Top vertical current conduction layer TVC 21600 is given by layer 11 and12 in Table 3-2 made up of InGaAsP layer that is composed of 25 nm-thicklayer 21600LP₂ that is P-doped with dopant density 21600P₂=1×10¹⁸/cm³,followed by 20 nm-thick N-doped InGaAsP layer 21600LN₂ with dopantdensity 21600N₂=1×10¹⁹/cm³. The total thickness for TVC 21600 is d_(TVC)21600 d with d_(TVC)=45 nm. Its width is W_(TVC) 21600 w equals toW_(TVC)=2 micrometers, and its averaged refractive index is n_(TVC)21600 n equals to n_(TVC)=3.4. This N₂P₂ junction forms a forward-BiasedPN Junction (or Tunnel PN Junction). It forms a PN-changing PN junction(called PNCPN junction) 21600PNCPN.

Top Interspaced Dielectric Current Conduction Layer

Top interspaced dielectric conduction layer TIDC 21650 is N-doped InPlayer given by layer 13 in Table 3-2 with thickness d_(TIDC) 21650 dequals to d_(TIDC)=20 nm, width w_(TIDC) 21650 w equals to w_(TIDC)=2micrometers, and averaged refractive index n_(TIDC) 21650 n equals ton_(TIDC)=3.0.

Top Vertical/Side Conduction and Ohmic Contact Layer

Top vertical/side conduction and Ohmic contact layer TVSCOC 21700 ismade up of Low-Refractive-Index Ohmic Transparent Conductor (LRI-OTC)(In₂O₃) given by layer 14 in Table 3-2 with thickness d_(TVSC) 21700 dequals to d_(TVSC)=240 nm, width w_(TVSC) 21700 w equals to w_(TVSC)=2micrometers, and an averaged refractive index n_(TVSC) 21700 n equals ton_(TVSC)=1.7.

Top Metal Contact Pads

The first top middle metal contact pad FTMM 21800M is multilayer metalmade up of (17 nm Au followed by 17 nm Ge followed by 17 nm Au followedby 17 nm Ni followed by 1000 nm Au) deposited on top of the top surfaceof n-doped layer 21700 given by layer 14 in Table 3-2. The totalthickness of the metal contact pad is d_(FTMM) 21800Md, withd_(FTMM)=1068 nm, and width w_(FTMM) 21800Mw, where w_(FTMM) isapproximately 2 micrometers. The length of the metal contact padg_(FTMM) 21800Mg is approximately 500 micrometers.

There is no top left or right metal contact pad FTLM 21800L or FTRM21800R.

Top Metal Electrodes

On top of the first top middle metal contact pad FTMM 21800M isdeposited the first top middle metal electrode FTMME 21130M which isgold of thickness of approximately 2 micrometer thick.

Beam Transport to Electro-Active Waveguiding Core Structure

Input tapering waveguide region between z1=z1ALS 22300 z 1ALS andz1=g_(ITWCo) 22300 g, Tapering waveguide core width w_(ITWCo-z) 22300 wvaries down to a smaller value of w_(ITWCo-g)=50 nm at z1=g_(ITWCo)22300 g from its vale at z1=z1 ALS 22300 z 1ALS. ClearlyW_(ITWCo-g)<<λ_(bm)/(2*n_(ITWCo)), with λ_(bm)=1550 nm andn_(ITWCo)=3.6, where * is number multiplication.

Output Connecting Waveguide

Output connecting waveguide core OCWCo 28200 has averaged RefractiveIndex n_(OCWCo)=n_(aOCWCo)=3.6, thickness d_(OCWCo) 28200 d isd_(OCWCo)=250 nm, and width W_(OCWCo) 28200 w is W_(OCWCo)=400 nm.

Output connecting waveguide OCWG 28200WG has Output connecting-waveguidebottom cladding material OCWBCd 28200B that is silicon dioxide (SiO₂)for which the refractive index n_(OCWBCd) 28200Bn is n_(OCWBCd)=1.45.

Output connecting waveguide top cladding material OCWTCd 28200T issilicon dioxide foe which the refractive index n_(OCWTCd) 28200Tn isn_(OCWTCd)=1.45.

Output connecting waveguide left cladding material OCWLCd 28200L issilicon dioxide for which the refractive index n_(OCWLCd) 28200Ln isn_(OCWLCd)=1.45.

Output connecting waveguide right cladding material OCWRCd 28200R issilicon dioxide for which the refractive index n_(OCWRCd) 28200Rn isn_(OCWRCd)=1.45.

The resulted averaged cladding refractive Index n_(aOCWCd) 28200 aCdn isn_(aOCWCd)=1.45.

Output Optical Beam OBM 28140

Output Beam Coupler Structure (OBCS) Region

Output tapering waveguide core OTWCo 28300 is made of silicon. Its widthat a location z2 OTWCo-z2 is denoted as width w_(OTWCo-z2) 28300 w-z 2.This width is tapered from width at z2=0 w_(OTWCo-z2=0) 28300 w-z 2=0that has a value of w_(OTWCo-z2=0)=400 nm to a width at z2>0w_(OTWCo-z2>0) 28300 w-z 2>0 that is narrower than 400 nm in a linearfashion.

The thickness of the tapering waveguide core d_(OTWCo-z2) 28300 d-z 2made of silicon is d_(OTWCo-z2)=250 nm with a refractive indexn_(OTWCo-z2) 28300 n-z 2 that is n_(OTWCo-z2)=3.6.

The total length of tapering waveguide g_(OTWCo) 28300 g is g_(OTWCo)=20micrometers (μm). The width of the waveguide core at the end of thetapering at z2=g_(OTWCo) is w_(OTWCo-g) 28300 w-g with w_(OTWCo-g)=50nm.

Output supporting structure OSTR 29200 has width w_(OSTR) 29200 w withw_(OSTR)=50 nm and thickness d_(OSTR) 29200 d with d_(OSTR)=250 nm andlength g_(OSTR) 29200 g with g_(OSTR)=20 micrometers. It has aneffective layer averaged refractive index n_(laOSTR) 29200 nla withn_(laOSTR)<2.5.

The top cladding OTWTCd-z2 28300T-z2 is silicon dioxide (SiO₂) hasrefractive index n_(OTWTCd-z2) 28300Tn-z2 with n_(OTWTCd-z2)=1.45 beforegoing into the ALS region.

The bottom cladding OTWBCd-z2 28300B-z2 is silicon dioxide (SiO₂) hasrefractive index n_(OTWBCd-z2) 28300Bn-z2 with n_(OTWBCd-z2)=1.45.

The left cladding OTWLCd-z2 28300L-z2 is silicon dioxide (SiO₂) hasrefractive index n_(OTWLCd-z2) 28300Ln-z2 with n_(OTWLCd-z2)=1.45.

The right cladding OTWRCd-z2 28300R-z2 is silicon dioxide (SiO₂) hasrefractive index n_(OTWRCd-z2) 28300Rn-z2 with n_(OTWRCd-z2)=1.45.

In this exemplary embodiment,n_(OTWTCd-z2)=n_(OTWBCd-z2)=n_(OTWLCd-z2)=n_(OTWRCd-z2)=n_(OCWTCd), andn_(OCWTCd)=n_(OCWBCd)=n_(OCWLCd)=n_(OCWRCd)

Output tapering waveguide core OTWCo 28300 starting at z2=z2ALS 28300 z2ALS, is laid with an active layer structure ALS 22500.0<z2ALS<g_(OTWCo).

Most of the output optical beam energy of beam OBM 28140 is transportedto output tapering waveguide core OTWCo 28300 from the electro-activewaveguiding core structure EWCoS 22600, through the output taperingwaveguide region between z2=z2ALS 28300 z 2ALS and z2=g_(OTWCo) 28300 g,where the output tapering waveguide core width w_(OTWCo-z2) 28300 w-z 2varies down to a smaller value of w_(OTWCo-g) at z2=g_(ITWCo) 28300 gfrom its vale at z2=z2ALS, 28300 z 2ALS. The tapering waveguide corewidth is reduced to well below half the optical wavelength in thewaveguide core given by λ_(bm)/(2×n_(OTWCo)) so thatw_(OTWCo-g)<<λ_(bm)/(2×n_(OTWCo)). After the energy transported from theelectro-active waveguiding core structure EWCoS 22600 that contains theelectro-active layer EC 21500 down to the output taper at z2=0 where thetaper core width is w_(OTWCo-z2=0) 28300 w 0 andw_(OTWCo-z2=0)=w_(OCWCo) 28200, the optical beam is denoted as outputoptical beam or beam OBM 28140.

Length of Active Layer Structure

The length of the active layer structure SL_(mod) 22550 is approximately500 micrometers.

High Refractive Index Contrast and Mode Overlapping

For the bottom cladding:Waveguide core refractive index is n_(BCo)=3.6Waveguide bottom cladding is n_(BCd)=1.45 (given by layer ISTRBC withn_(ISTRBCd)=1.45)Waveguide core-to-cladding refractive index difference square to ben_(rd) ²=(n_(co) ²=n_(BCd) ²)=10.86.Refractive index contrast ratio to be: R_(cts)=n_(rd) ²/(n_(co)²+n_(BCd) ²)=0.7, which is in the very-strongly guiding regime.For the top cladding:Waveguide core refractive index is n_(co)=3.6Waveguide bottom cladding is n_(TCd)=1.7 (given by TVSCOC layer which isIn₂O₃ with n_(TVSCOC)=1.7)Waveguide core-to-cladding refractive index difference square to ben_(rd) ²=(n_(co) ²−n_(TCd) ²)=10.Refractive index contrast ratio to be: R_(cts)=n_(rd) ²/(n_(co)²+n_(TCd) ²)=0.64, which is in the very-strongly guiding regime.A Third Exemplary Device of Electro-Optic Modulator with TransparentConductor Geometry

A preferred embodiment of an exemplary device is Modulator Device 20000with the following specifications referred to as a third exemplarydevice of electro-optic modulator with Ohmic transparent conductorgeometry: The main difference between this and the Second ExemplaryDevice is in Table 3-3, in which the active layer is designed forelectro-optic modulation.

Substrate SUB 21100 is silicon wafer substrate with a thickness of about0.3 mm.

Input connecting waveguide gore ICWCo 22200 is made of silicon for whichits averaged material refractive index n_(ICWCo) 22200 n is aroundn_(ICWCo)=3.6, thickness d_(ICWCo) 22200 d is d_(ICWCo)=250 nm, andwidth W_(ICWCo) 22200 w is W_(ICWCo)=400 nm.

Input connecting-waveguide bottom cladding material ICWBCd 22200B issilicon dioxide (SiO₂), for which its refractive Index n_(ICWBCd)22200Bn is n_(ICWBCd)=1.45.

Input connecting waveguide top cladding material ICWTCd 22200T issilicon dioxide (SiO₂) for which its refractive Index n_(ICWTCd) 22200Tnis 1.45.

Input connecting waveguide left cladding material ICWLCd 22200L issilicon dioxide (SiO₂), for which its refractive Index n_(ICWLCd)22200Ln is 1.45

Input connecting waveguide right cladding material ICWRCd 22200R issilicon dioxide (SiO₂), for which its refractive Index n_(ICWRCd)22200Rn is 1.45

The above form an input connecting waveguide ICWG 22200WG. Thecore-cladding refractive-index difference n_(Rd) defined by n_(Rd)²=(n_(Co) ²−n_(Cd) ²) for waveguide ICWG 22200WG is n_(Rd)²=(3.6²−1.45²)=10.86 with n_(Co)=3.6 and n_(Cd)=1.45. Its averagedCladding Refractive Index is given by n_(aICWCd)=(n_(ICWBCd)²×A_(ICWBCd)+n_(ICWTCd) ²×A_(ICWTCd)+n_(ICWRCd) ² A_(ICWRCd)+n_(ICWLCd)² A_(ICWLCd))/(A_(ICWBCd)+A_(ICWTCd)+A_(ICWRCd)+A_(ICWLCd))^(0.5)=1.45.Its averaged Core Refractive Index is given by n_(aCo)=(n_(Co1)²×A_(Co1)+n_(Co2) ²×A_(Co2)+n_(Co3) ² A_(Co3)+ . . . +n_(Com) ²A_(Com))/(A_(Co1)+A_(Co2)+A_(Co3)+ . . . +A_(Com))^(0.5)=3.6.

The input optical beam IBM 22140 has propagating refractive indexn_(IBM) 22140 n, for which n_(IBM) is approximately 2.8 with opticalpower P_(bm) 22140P approximately 1 mW, electric field polarizationE_(bm) 22140E to be in the horizontal direction parallel to thesubstrate surface. It has a beam effective area A_(bm) 22140A ofA_(bm)=0.04 μm² and an optical wavelength centered at λ_(bm) 22140L withλ_(bm)=1550 nm with plurality of (N) frequency channels λ_(bm1)=1548 nm,λ_(bm2)=1549 nm, λ_(bm3)=1550 nm, λ_(bm4)=1551 nm, and λ_(bm3)=1552 nmcentered at λ_(bm)=1550 nm.

Input Beam Coupler Structure (IBCS) Region

The input tapering waveguide core ITWCo 223000 is made of silicon. Itswidth at a location z1, ITWCo-z1 22300 z 1 is denoted as widthw_(ITWCo-z1) 22300 w-z 1. This width is tapered from width at z1=0w_(ITWCo-z1=0) 22300 w-z 1=0 that has a value of w_(ITWCo-z1=0)=400nanometers (nm) to a width at z1>0 w_(ITWCo-z1>0) 22300 w-z 1>0 that isnarrower than 400 nm in a linear fashion.

The thickness of the tapering waveguide core d_(ITWCo-z1) 22300 d-z 1made of silicon is d_(ITWCo-z1)=250 nm with a refractive indexn_(ITWCo-z1) 22300 n-z that is n_(ITWCo-z1)=3.6.

The total length of tapering waveguide g_(ITWCo) 22300 g is g_(ITWCo)=20micrometers (μm). The width of the waveguide core at the end of thetapering at z1=g_(ITWCo) is w_(ITWCo-g) 22300 w-g with w_(ITWCo-g)=50nm.

Input supporting structure ISTR 21200 has width w_(ISTR) 21200 w withw_(ISTR)=50 nm and thickness d_(ISTR) 21200 d with d_(ISTR)=250 nm andlength g_(ISTR) 21200 g with g_(ISTR)=20 micrometers. It has aneffective layer averaged refractive index n_(laISTR) 21200 nla withn_(laISTR)<2.5.

Left cladding material ISTRLCd 21200L is air and has a refractive indexn_(ISTRLCd) 21200Ln given by n_(ISTRLCd)=1, and Right cladding materialISTRRCd 21200R is air and has a refractive index n_(ISTRRCd) 21200Rngiven by n_(ISTRRCd)=1. Its bottom cladding ISTRBCd 21200B is silicondioxide (this is part of a Burried-Oxide BOX layer in a typicalSilicon-On-Insulator SOI wafer) with averaged refractive indexn_(ISTRBCd) 21200Bn of n_(ISTRBCd)=1.45.

The top cladding ITWTCd-z1 22300T-z1 before going into the ALS region issilicon dioxide (SiO₂) has refractive index n_(ITWTCd-z1) 22300Tn-z1with n_(ITWTCd-z1)=1.45.

The bottom cladding ITWBCd-z1 22300B-z1 before going into the ALS regionis silicon dioxide (SiO₂) has refractive index n_(ITWBCd-z1) 22300Bn-z1with n_(ITWBCd-z1)=1.45.

The left cladding ITWLCd-z1 22300L-z1 before going into the ALS regionis silicon dioxide (SiO₂) has refractive index n_(ITWLCd-z1) 22300Ln-z1with n_(ITWLCd-z1)=1.45.

The right cladding ITWRCd-z1 22300R-z1 before going into the ALS regionis silicon dioxide (SiO₂) has refractive index n_(ITWRCd-z1) 22300Rn-z1with n_(ITWRCd-z1)=1.45.

In this exemplary embodiment,n_(ITWTCd-z1)=n_(ITWBCd-z1)=n_(ITWLCd-z1)=n_(ITWRCd-z1)=n_(ICWTCd), andn_(ICWTCd)=n_(ICWBCd)=n_(ICWLCd)=n_(ICWRCd).mInput tapering waveguidecore ITWCo 22300 starting at z1=z1ALS 22300 z 1ALS, where z1ALS=10micrometers, is laid with an active layer structure ALS 22500.0<z1ALS<g_(ITWCo).

Active Layer Structure-Beam Transport into the Structure

The active layer structure ALS 22500 is shown by the Table 3-3 below:

TABLE 3-3 ALS 22500 Layer Doping/ Layer Number Thickness NPNN TCO CASE(1/cm³) BIM 100 nm  In₂O₃ (21250) BSCOC 1 100 nm  InGaAsP 1.3 um N = 1 ×(21300) (Bottom layer-just 10¹⁹ above the substrate) BIDC 2 40 nm InP N= 1 × (21350LN) 10¹⁹ BVC 3 20 nm InGaAsP 1.3 um N = 1 × (21400) 10¹⁹ EC4 10 nm AlGaInAs 1.3 um N₁ = 1 × (21500LN₁) 10¹⁹ EC 5 4 nm barrierAlGaInAs/1.1 um/−0.8% MN₁ = 4 × (21500MLN₁) tensile strained 10¹⁷ EC 6 2× 7 nm AlGaInAs/1.1 um/−0.8% MN₂ = 4 × (21500MLN₂) barrier insidetensile strained 10¹⁷ EC 7 3 × 6.5 nm AlGaInAs/1.55 um/0.9% MN₃ = 4 ×(21500MLN₃) Well (PL = compressive strained 10¹⁷ 1350 nm) EC 8 4 nmbarrier AlGaInAs/1.1 um/−0.8% MN₄ = 4 × (21500MLN₄) tensile strained10¹⁷ EC 9 43 nm AlGaInAs 1.3 um MN₅ = 4 × (21500MLN₅) 10¹⁷ EC 10 20 nmAlGaInAs 1.3 um P₁ = 1 × (21500LP₁) 10¹⁸ TVC 11 25 nm InGaAsP 1.3 um P₂= 1 × (21600P₂) 10¹⁸ TVC 12 20 nm InGaAsP 1.3 um N₂ = 1 × (21600N₂) 10¹⁹TIDC 13 20 nm InP N = 1 × (21650) 10¹⁹ TVSCOC 14 240 nm  In₂O₃ (Toplayer) N = 1 × (21700) 10¹⁹ Total 580 nm In the table, the materials are unstrained (with InP as the substrate)if not specified as strained. The wavelength specified will be thematerial bandgap wavelength of the quaternary material involved (properchoice of the material composition is needed to achieve the requiredmaterial bandgap and strain when grown on InP substrate).

Bottom Side Conduction and Ohmic Contact Layer

The active layer structure ALS 22500 has a bottom side conduction andOhmic contact layer BSCOC 21300 that is InGaAsP layer given by layer 1in Table 3-3 with thickness d_(BSC) 21300 d, where d_(BSC)=100 nm andwidth w_(BSC) 21300 w, where w_(BSC) is approximately 54 micrometersalong most of the length of the ALS. Its refractive index n_(BSC) 21300n is n_(BSC)=3.4.

Bottom Interspaced Material Layer

The bottom interspaced material layer BIM 21250 is made of aLow-Refractive-Index Ohmic Transparent Conducting (LRI-OTC) materialcomposed of Indium oxide (In₂O₃) with thickness d_(BIM) 21250 d equalsto d_(BIM)=100 nm, width w_(BIM) 21250 w equals to w_(BIM)=54micrometers, and average refractive index n_(BIM) 21250 n equals ton_(BIM)=1.7.

Bottom Metal Contact Pads

The first bottom left metal contact pad FBLM 21900L is a multi-layermetal made up of (17 nm Au followed by 17 nm Ge followed by 17 nm Aufollowed by 17 nm Ni followed by 1000 nm Au) deposited on top of the topsurface of n-doped layer 21300 given by layer 1 in Table 3-3. The totalthickness of the metal contact pad is d_(FBLM) 21900Ld, withd_(FBLM)=1068 nm, and width w_(FBLM) 21900Lw, where w_(FBLM) isapproximately 20 micrometers. The length of the metal contact padg_(FBLM) 21900Lg is approximately 500 micrometers.

The first bottom right metal contact pad FBRM 21900R is multilayer metalmade up of (17 nm Au followed by 17 nm Ge followed by 17 nm Au followedby 17 nm Ni followed by 100 nm Au) deposited on top of the top surfaceof n-doped layer 21300 given by layer 1 in Table 3-3. The totalthickness of the metal contact pad is d_(FBRM) 21900Rd, withd_(FBRM)=1068 nm, and width w_(FBRM) 21900Rw, where w_(FBRM) isapproximately 20 micrometers. The length of the metal contact padg_(FBRM) 21900Rg is approximately 500 micrometers.

Bottom Metal Electrodes

On top of the first bottom left metal contact pad FBLM 21900L isdeposited the first bottom left metal electrode FBLME 21120L which isgold of thickness of approximately 2 micrometer thick.

On top of the first bottom right metal contact pad FBRM 21900R isdeposited the first bottom right metal electrode FBRME 21120R which isgold of thickness of approximately 2 micrometer thick.

Bottom Interspaced Dielectric Current Conduction Layer

Bottom interspaced dielectric current conduction layer BIDC 21350 is an-doped InP given by layer 2 in Table 3-3 with thickness d_(BIDC) 21350d equals to d_(BIDC)=40 nm, width w_(BIDC) 21350 w equals to w_(BIDC)=54micrometers, and average refractive index n_(BIDC) 21350 n equals toabout n_(BIDC)=3.0.

Bottom Vertical Current Conduction Layer

Bottom vertical current conduction layer BVC 21400 is n-doped InGaAsPgiven by layer 3 in Table 3-3 with thickness d_(BVC) 21400 d equals tod_(BVC)=20 nm, width w_(BVC) 21400 w equals to w_(BVC)=2 micrometers,and an averaged refractive index nave 21400 n equals to n_(BVC)=3.4.

Electro-Active Layer

Electro-active layer EC 21500 is made up of layers 4, 5, 6, 7, 8, 9, 10in Table 3-1 with an averaged refractive index of the entire layer givenby n_(EC) 21500 n with n_(EC) equals to approximately n_(EC)=3.4. Underan applied electric field, there will be a change in averaged refractiveindex dn_(EC) 21500 dn. The average refractive index becomesn_(EC)(new)=n_(EC)+dn_(EC).

The total thickness d_(EC) 21500 d of this Electro-active layer isd_(EC)=160 nm. Its width w_(EC) 21500 w is equal to w_(EC)=2micrometers.

The electro-active layer has a PqN junction at layer 4 to 10 for whichlayer 4 is layer 21500LN₁ that is N-doped with a dopant density of21500N₁=1×10¹⁹/cm³ and layer 10 is layer 21500LP₁ that is P-doped with adopant density of 21500P₁=1×10¹⁸/cm³

The intermediate layers 21500MLN_(m) are all N-doped with a dopantdensity of 21500MN_(m)=4×10¹⁷/cm³.

The applied field E_(EC) 21500E (which may cause a current C_(EC) 21500Cto flow) is across the entire electro-active layer with a negativevoltage applied to the top and positive voltage applied to the bottom ofthis entire electro-active layer known to those skilled in the art asrevered bias (with respect to the PN junction in the electro-activelayer) of voltage V_(R) 21500VR so the applied electro-active V_(EC)21500VEC is V_(R).

The voltage applied to the electrodes of the modulator V_(MOD) 20000V isapproximately given by V_(EC).

Top Vertical Current Conduction Layer

Top vertical current conduction layer TVC 21600 is given by layer 11 and12 in Table 3-3 made up of InGaAsP layer that is composed of 25 nm-thicklayer 21600LP₂ that is P-doped with dopant density 21600P₂=1×10¹⁸/cm³,followed by 20 nm-thick N-doped InGaAsP layer 21600LN₂ with dopantdensity 21600N₂=1×10¹⁹/cm³. The total thickness for TVC 21600 is d_(TVC)21600 d with d_(TVC)=45 nm. Its width is W_(TVC) 21600 w equals toW_(TVC)=2 micrometers, and its averaged refractive index is n_(TVC)21600 n equals to n_(TVC)=3.4. This N₂P₂ junction forms a forward-BiasedPN Junction (or Tunnel PN Junction). It forms a PN-changing PN junction(called PNCPN junction) 21600PNCPN.

Top Interspaced Dielectric Current Conduction Layer

Top interspaced dielectric conduction layer TIDC 21650 is N-doped InPlayer given by layer 13 in Table 3-3 with thickness d_(TIDC) 21650 dequals to d_(TIDC)=20 nm, width w_(TIDC) 21650 w equals to w_(TIDC)=2micrometers, and averaged refractive index n_(TIDC) 21650 n equals ton_(TIDC)=3.0.

Top Vertical/Side Conduction and Ohmic Contact Layer

Top vertical/side conduction and Ohmic contact layer TVSCOC 21700 ismade up of Low-Refractive-Index Ohmic Transparent Conductor (LRI-OTC)(In₂O₃) given by layer 14 in Table 3-3 with thickness d_(TVSC) 21700 dequals to d_(TVSC)=240 nm, width w_(TVSC) 21700 w equals to w_(TVSC)=2micrometers, and an averaged refractive index n_(TVSC) 21700 n equals ton_(TVSC)=1.7.

Top Metal Contact Pads

The first top middle metal contact pad FTMM 21800M is multilayer metalmade up of (17 nm Au followed by 17 nm Ge followed by 17 nm Au followedby 17 nm Ni followed by 1000 nm Au) deposited on top of the top surfaceof n-doped layer 21700 given by layer 14 in Table 3-3. The totalthickness of the metal contact pad is d_(FTMM) 21800Md, withd_(FTMM)=1068 nm and width w_(FTMM) 21800Mw, where w_(FTMM) isapproximately 2 micrometers. The length of the metal contact padg_(FTMM) 21800Mg is approximately 500 micrometers.

There is no top left or right metal contact pad FTLM 21800L or FTRM21800R.

Top Metal Electrodes

On top of the first top middle metal contact pad FTMM 21800M isdeposited the first top middle metal electrode FTMME 21130M which isgold of thickness of approximately 2 micrometer thick.

Beam Transport to Electro-Active Waveguiding Core Structure

Input tapering waveguide region between z1=z1ALS 22300 z 1ALS andz1=g_(ITWCo) 22300 g, Tapering waveguide core width w_(ITWCo-z1) 22300 wvaries down to a smaller value of w_(ITWCo-g)=50 nm at z1=g_(ITWCo)22300 g from its vale at z1=z1ALS 22300 z 1ALS.

Clearly W_(ITWCo-g)<<λ_(bm)(2*n_(ITWCo)), with λ_(bm)=1550 nm andn_(ITWCo)=3.6, where * is number multiplication.

Output Connecting Waveguide

Output connecting waveguide core OCWCo 28200 has averaged RefractiveIndex n_(OCWCo)=n_(aOCWCo)=3.6, thickness d_(OCWCo) 28200 d isd_(OCWCo)=250 nm, and width W_(OCWCo) 28200 w is W_(OCWCo)=400 nm.

Output connecting waveguide OCWG 28200WG has Output connecting-waveguidebottom cladding material OCWBCd 28200B that is silicon dioxide (SiO₂)for which the refractive index n_(OCWBCd) 28200Bn is n_(OCWBCd)=1.45.

Output connecting waveguide top cladding material OCWTCd 28200T issilicon dioxide foe which the refractive index n_(OCWTCd) 28200Tn isn_(OCWTCd)=1.45.

Output connecting waveguide left cladding material OCWLCd 28200L issilicon dioxide for which the refractive index n_(OCWLCd) 28200Ln isn_(OCWLCd)=1.45.

Output connecting waveguide right cladding material OCWRCd 28200R issilicon dioxide for which the refractive index n_(OCWRCd) 28200Rn isn_(OCWRCd)=1.45.

The resulted averaged cladding refractive Index n_(aOCWCd) 28200 aCdn isn_(aOCWCd)=1.45.

Output Optical Beam OBM 28140

Output Beam Coupler Structure (OBCS) Region

Output tapering waveguide core OTWCo 28300 is made of silicon. Its widthat a location z2 OTWCo-z2 is denoted as width w_(OTWCo-z2) 28300 w-z 2.This width is tapered from width at z2=0 w_(OTWCo-z2=0) 28300 w-z 2=0that has a value of w_(OTWCo-z2=0)=400 nm to a width at z2>0w_(OTWCo-z2) 28300 w-z 2>0 that is narrower than 400 nm in a linearfashion.

The thickness of the tapering waveguide core d_(OTWCo-z2) 28300 d-z 2made of silicon is d_(OTWCo-z2)=250 nm with a refractive indexn_(OTWCo-z2) 28300 n-z 2 that is n_(OTWCo-z2)=3.6.

The total length of tapering waveguide g_(OTWCo) 28300 g is g_(OTWCo)=20micrometers (□m). The width of the waveguide core at the end of thetapering at z2=g_(OTWCo) is w_(OTWCo-g) 28300 w-g with w_(OTWCo-g)=50nm.

Output supporting structure OSTR 29200 has width w_(OSTR) 29200 w withw_(OSTR)=50 nm and thickness d_(OSTR) 29200 d with d_(OSTR)=250 nm andlength g_(OSTR) 29200 g with g_(OSTR)=20 micrometers. It has aneffective layer averaged refractive index n_(laOSTR) 29200 nla withn_(laOSTR)<2.5.

The top cladding OTWTCd-z2 28300T-z2 is silicon dioxide (SiO₂) hasrefractive index n_(OTWTCd-z2) 28300Tn-z2 with n_(OTWTCd-z2)=1.45 beforegoing into the ALS region.

The bottom cladding OTWBCd-z2 28300B-z2 is silicon dioxide (SiO₂) hasrefractive index n_(OTWBCd-z2) 28300Bn-z2 with n_(OTWBCd-z2)=1.45.

The left cladding OTWLCd-z2 28300L-z2 is silicon dioxide (SiO₂) hasrefractive index n_(OTWLCd-z2) 28300Ln-z2 with n_(OTWLCd-z2)=1.45.

The right cladding OTWRCd-z2 28300R-z2 is silicon dioxide (SiO₂) hasrefractive index n_(OTWRCd-z2) 28300Rn-z2 with n_(OTWRCd-z2)=1.45.

In this exemplary embodiment,n_(OTWTCd-z2)=n_(OTWBCd-z2)=n_(OTWLCd-z2)=n_(OTWRCd-z2)=n_(OCWTCd), andn_(OCWTCd)=n_(OCWBCd)=n_(OCWLCd)=n_(OCWRCd).

Output tapering waveguide core OTWCo 28300 starting at z2=z2ALS 28300 z2ALS, is laid with an active layer structure ALS 22500.0<z2ALS<g_(OTWCo).

Most of the output optical beam energy of beam OBM 28140 is transportedto output tapering waveguide core OTWCo 28300 from the electro-activewaveguiding core structure EWCoS 22600, through the output taperingwaveguide region between z2=z2ALS 28300 z 2ALS and z2=g_(OTWCo) 28300 g,where the output tapering waveguide core width w_(OTWCo-z2) 28300 w-z 2varies down to a smaller value of w_(OTWCo-g) at z2=g_(ITWCo) 28300 gfrom its vale at z2=z2ALS. 28300 z 2ALS. The tapering waveguide corewidth is reduced to well below half the optical wavelength in thewaveguide core given by λ_(bm)/(2×n_(OTWCo)) so thatw_(OTWCo-g)<<λ_(bm)/(2×n_(OTWCo)). After the energy transported from theelectro-active waveguiding core structure EWCoS 22600 that contains theelectro-active layer EC 21500 down to the output taper at z2=0 where thetaper core width is w_(OTWCo-z2=0) 28300 w 0 andw_(OTWCo-z2=0)=w_(OTWCo) 28200, the optical beam is denoted as outputoptical beam or beam OBM 28140.

Length of Active Layer Structure

The length of the active layer structure SL_(mod) 22550 is approximately500 micrometers.

High Refractive Index Contrast and Mode Overlapping

For the bottom cladding:Waveguide core refractive index is n_(co)=3.6Waveguide bottom cladding is n_(BCd)=1.45 (given by layer ISTRBC withn_(ISTRBCd)=1.45)Waveguide core-to-cladding refractive index difference square to ben_(rd) ²=(n_(co) ²−n_(BCd) ²)=10.86.Refractive index contrast ratio to be: R_(cts)=n_(rd) ²/(n_(co)²+n_(BCd) ²)=0.7, which is in the very-strongly guiding regime.For the top cladding:Waveguide core refractive index is n_(co)=3.6Waveguide bottom cladding is n_(TCd)=1.7 (given by TVSCOC layer which isIn₂O₃ with n_(TVSCOC)=1.7)Waveguide core-to-cladding refractive index difference square to ben_(rd) ²=(n_(co) ²−n_(TCd) ²)=10.Refractive index contrast ratio to be: R_(cts)=n_(rd) ²/(n_(co)²+n_(TCd) ²²)=0.64, which is in the very-strongly guiding regime.

Final Summary

The main parts of the embodiments can be summarized as follows:

-   -   1. A lowloss-low-voltage-high-frequency optical phase or        intensity modulator device deposed on a substrate. The device        has at least an input connecting waveguide core deposed on the        substrate connecting the energy of an optical beam to and from        an electro-active layer. The optical beam has one or more        optical wavelengths around an operating optical wavelength        □_(op).        -   The input connecting waveguide core becomes an input            tapering waveguide core and enters below an electro-active            layer. The optical beam energy is well-confined in the input            tapering waveguide core before the tapering waveguide core            enters below the electro-active layer. The optical beam            energy is no longer well-confined in the input tapering            waveguide core at some point after the tapering waveguide            core enters below the electro-active layer.        -   The refractive index n_(EC) or the optical gain/absorption            coefficient □_(EC) of at least part of the material in the            electro-active layer can be altered by an applied electric            field, an electric current, or either injection or depletion            of carriers in the electro-active layer. The electro-active            layer is either part of or in spatial proximity to an            electro-active waveguide core.        -   The electro-active waveguide core and electro-active            waveguide cladding structure is in a medium-strongly guiding            or very-strongly guiding regime such that the refractive            index contrast of the waveguide core layer with both the top            and the bottom waveguide cladding defined by:            R_(cts)=(n_(Co) ²−n_(Cd) ²)/(n_(Co) ²+n_(Cd) ²) are both            larger than about 0.2, where n_(Cd) is the averaged material            refractive index of either the top or the bottom waveguide            cladding region, and n_(Co) is the averaged material            refractive index of the waveguide core region.        -   The electro-active waveguide core thickness d_(CORE) is in            the ultra-thin, very-thin, medium-thin, or thin region such            that d_(CORE)<(2*λ_(op)/n_(Co)).    -   2. A lowloss-low-voltage-high-frequency optical phase or        intensity modulator device deposed on a substrate. The device        has at least an input connecting waveguide core deposed on the        substrate connecting the energy of an optical beam to and from        an electro-active layer. The optical beam has one or more        optical wavelengths around an operating optical wavelength        λ_(op).        -   The input connecting waveguide core becomes an input            tapering waveguide core and enters below an electro-active            layer. The optical beam energy is well-confined in the input            tapering waveguide core before the tapering waveguide core            enters below the electro-active layer. The optical beam            energy is no longer well-confined in the input tapering            waveguide core at some point after the tapering waveguide            core enters below the electro-active layer.        -   The refractive index n_(EC) or the optical gain/absorption            coefficient □_(EC) of at least part of the material in the            electro-active layer can be altered by an applied electric            field, an electric current, or either injection or depletion            of carriers in the electro-active layer. The electro-active            layer is either part of or in spatial proximity to an            electro-active waveguide core.        -   The electro-active waveguide core and electro-active            waveguide cladding structure is in a medium-strongly guiding            or very-strongly guiding regime such that the refractive            index contrast of the waveguide core layer with both the top            and the bottom waveguide cladding defined by:            R_(cts)=(n_(Co) ²−n_(Cd) ²)/(n_(Co) ²+n_(Cd) ²) are both            larger than about 0.2, where n_(Cd) is the averaged material            refractive index of either the top or the bottom waveguide            cladding region, and n_(Co) is the averaged material            refractive index of the waveguide core region.        -   The electro-active waveguide core thickness d_(CORE) is in            the ultra-thin, very-thin, medium-thin, or thin region such            that d_(CORE)<(2*λ_(op)/n_(Co)).        -   The electro-active layer has a low-refractive-index Ohmic            transparent conductor (LRI-OTC) layer electrically connected            from the top to the electro-active layer. The LRI-OTC forms            part of the top electro-active waveguide cladding.    -   3. A lowloss-low-voltage-high-frequency optical phase or        intensity modulator device deposed on a substrate. The device        has at least an input connecting waveguide core deposed on the        substrate connecting the energy of an optical beam to and from        an electro-active layer. The optical beam has one or more        optical wavelengths around an operating optical wavelength        λ_(op).        -   The input connecting waveguide core becomes an input            tapering waveguide core and enters below an electro-active            layer. The optical beam energy is well-confined in the input            tapering waveguide core before the tapering waveguide core            enters below the electro-active layer. The optical beam            energy is no longer well-confined in the input tapering            waveguide core at some point after the tapering waveguide            core enters below the electro-active layer.        -   The refractive index n_(EC) or the optical gain/absorption            coefficient α_(EC) of at least part of the material in the            electro-active layer can be altered by an applied electric            field, an electric current, or either injection or depletion            of carriers in the electro-active layer. The electro-active            layer is either part of or in spatial proximity to an            electro-active waveguide core.        -   The electro-active waveguide core and electro-active            waveguide cladding structure is in a medium-strongly guiding            or very-strongly guiding regime such that the refractive            index contrast of the waveguide core layer with both the top            and the bottom waveguide cladding defined by:            R_(cts)=(n_(Co) ²−n_(Cd) ²)/(n_(Co) ²+n_(Cd) ²) are both            larger than about 0.2, where n_(Cd) is the averaged material            refractive index of either the top or the bottom waveguide            cladding region, and n_(Co) is the averaged material            refractive index of the waveguide core region.        -   The electro-active waveguide core thickness d_(CORE) is in            the ultra-thin region or very-thin such that            d_(CORE)<(λ_(op)/n_(Co)).    -   4. A lowloss-low-voltage-high-frequency optical phase or        intensity modulator device deposed on a substrate. The device        has at least an input connecting waveguide core deposed on the        substrate connecting the energy of an optical beam to and from        an electro-active layer. The optical beam has one or more        optical wavelengths around an operating optical wavelength        λ_(op).        -   The input connecting waveguide core becomes an input            tapering waveguide core and enters below an electro-active            layer. The optical beam energy is well-confined in the input            tapering waveguide core before the tapering waveguide core            enters below the electro-active layer. The optical beam            energy is no longer well-confined in the input tapering            waveguide core at some point after the tapering waveguide            core enters below the electro-active layer.        -   The refractive index n_(EC) or the optical gain/absorption            coefficient α_(EC) of at least part of the material in the            electro-active layer can be altered by an applied electric            field, an electric current, or either injection or depletion            of carriers in the electro-active layer. The electro-active            layer is either part of or in spatial proximity to an            electro-active waveguide core.        -   The electro-active waveguide core and electro-active            waveguide cladding structure is in an very-strongly guiding            regime such that the refractive index contrast of the            waveguide core layer with both the top and the bottom            waveguide cladding defined by: R_(cts)=(n_(Co) ²−n_(Cd)            ²)/(n_(Co) ²+n_(Cd) ²) are both larger than about 0.5, where            n_(Cd) is the averaged material refractive index of either            the top or the bottom waveguide cladding region, and n_(Co)            is the averaged material refractive index of the waveguide            core region.        -   The electro-active waveguide core thickness d_(CORE) is in            the ultra-thin or very-thin region such that            d_(CORE)<(λ_(op)/n_(Co)).

(PqN Case)

-   -   5. A lowloss-low-voltage-high-frequency optical phase or        intensity modulator device deposed on a substrate. The device        has at least an input connecting waveguide core deposed on the        substrate connecting the energy of an optical beam to and from        an electro-active layer. The optical beam has one or more        optical wavelengths around an operating optical wavelength        λ_(op).        -   The input connecting waveguide core becomes an input            tapering waveguide core and enters below an electro-active            layer. The optical beam energy is well-confined in the input            tapering waveguide core before the tapering waveguide core            enters below the electro-active layer. The optical beam            energy is no longer well-confined in the input tapering            waveguide core at some point after the tapering waveguide            core enters below the electro-active layer.        -   The refractive index n_(EC) or the optical gain/absorption            coefficient α_(EC) of at least part of the material in the            electro-active layer can be altered by an applied electric            field, an electric current, or either injection or depletion            of carriers in the electro-active layer. The electro-active            layer is either part of or in spatial proximity to an            electro-active waveguide core.        -   The electro-active waveguide core and electro-active            waveguide cladding structure is in a medium-strongly guiding            or very-strongly guiding regime such that the refractive            index contrast of the waveguide core layer with both the top            and the bottom waveguide cladding defined by:            R_(cts)=(n_(Co) ²−n_(Cd) ²)/(n_(Co) ²+n_(Cd) ²) are both            larger than about 0.2, where n_(Cd) is the averaged material            refractive index of either the top or the bottom waveguide            cladding region, and n_(Co) is the averaged material            refractive index of the waveguide core region.        -   The electro-active waveguide core thickness d_(CORE) is in            the ultra-thin, very-thin, medium-thin, or thin region such            that d_(CORE)<(2*λ_(op)/n_(Co)).        -   A structure electrically connected to the electro-active            layer comprises at least a first PN junction in which a            first P-layer with P-dopant is vertically connected            (vertical means in a direction perpendicular to the            substrate plane: horizontal means in a direction parallel to            the substrate plane) to a first N-layer with N-dopant, or a            PqN junction in which a first P-layer with P-dopant is            connected to a first q-layer with either N or P dopant or            that is undoped (i.e. being an Intrinsic semiconductor            material) and the q-layer is further connected to a first            N-layer with N-dopant.        -   A voltage is applied across the first P-layer of this first            PN junction, and the first N-layer of the first PN junction            to result in an applied electric field, an electric current,            or either injection or depletion of carriers in the            electro-active layer.    -   6. (PqN case plus Tunnel) A lowloss-low-voltage-high-frequency        optical phase or intensity modulator device deposed on a        substrate. The device has at least an input connecting waveguide        core deposed on the substrate connecting the energy of an        optical beam to and from an electro-active layer. The optical        beam has one or more optical wavelengths around an operating        optical wavelength λ_(op).        -   The input connecting waveguide core becomes an input            tapering waveguide core and enters below an electro-active            layer. The optical beam energy is well-confined in the input            tapering waveguide core before the tapering waveguide core            enters below the electro-active layer. The optical beam            energy is no longer well-confined in the input tapering            waveguide core at some point after the tapering waveguide            core enters below the electro-active layer.        -   The refractive index n_(EC) or the optical gain/absorption            coefficient α_(EC) of at least part of the material in the            electro-active layer can be altered by an applied electric            field, an electric current, or either injection or depletion            of carriers in the electro-active layer. The electro-active            layer is either part of or in spatial proximity to an            electro-active waveguide core.        -   The electro-active waveguide core and electro-active            waveguide cladding structure is in a medium-strongly guiding            or very-strongly guiding regime such that the refractive            index contrast of the waveguide core layer with both the top            and the bottom waveguide cladding defined by:            R_(cts)=(n_(Co) ²−n_(Cd) ²)/(n_(Co) ²+n_(Cd) ²) are both            larger than about 0.2, where n_(Cd) is the averaged material            refractive index of either the top or the bottom waveguide            cladding region, and n_(Co) is the averaged material            refractive index of the waveguide core region.        -   The electro-active waveguide core thickness d_(CORE) is in            the ultra-thin, very-thin, medium-thin, or thin region such            that d_(CORE)<(2*λ_(op)/n_(Co)).        -   A structure electrically connected to the electro-active            layer comprises at least a first PN junction in which a            first P-layer with P-dopant is vertically connected            (vertical means in a direction perpendicular to the            substrate plane; horizontal means in a direction parallel to            the substrate plane) to a first N-layer with N-dopant, or a            PqN junction in which a first P-layer with P-dopant is            connected to a first q-layer with either N or P dopant or            that is undoped (i.e. being an Intrinsic semiconductor            material) and the q-layer is further connected to a first            N-layer with N-dopant.        -   The first P-layer is electrically connected to a second            P-layer with P-dopant of a second PN junction, referred to            as the PN-changing PN junction (PNCPN). This second P-layer            is electrically connected to a second N-layer with N-dopant            of this second PN junction.        -   A voltage is applied across the second N-layer of the second            PN junction, and the first N-layer of the first PN junction            to result in an applied electric field, an electric current,            or either injection or depletion of carriers in the            electro-active layer.    -   7. (NqN case) A lowloss-low-voltage-high-frequency optical phase        or intensity modulator device deposed on a substrate. The device        has at least an input connecting waveguide core deposed on the        substrate connecting the energy of an optical beam to and from        an electro-active layer. The optical beam has one or more        optical wavelengths around an operating optical wavelength        λ_(op).        -   The input connecting waveguide core becomes an input            tapering waveguide core and enters below an electro-active            layer. The optical beam energy is well-confined in the input            tapering waveguide core before the tapering waveguide core            enters below the electro-active layer. The optical beam            energy is no longer well-confined in the input tapering            waveguide core at some point after the tapering waveguide            core enters below the electro-active layer.        -   The refractive index n_(EC) or the optical gain/absorption            coefficient α_(EC) of at least part of the material in the            electro-active layer can be altered by an applied electric            field, an electric current, or either injection or depletion            of carriers in the electro-active layer. The electro-active            layer is either part of or in spatial proximity to an            electro-active waveguide core.        -   The electro-active waveguide core and electro-active            waveguide cladding structure is in a medium-strongly guiding            or very-strongly guiding regime such that the refractive            index contrast of the waveguide core layer with both the top            and the bottom waveguide cladding defined by:            R_(cts)=(n_(Co) ²−n_(Cd) ²)/(n_(Co) ²+n_(Cd) ²) are both            larger than about 0.2, where n_(Cd) is the averaged material            refractive index of either the top or the bottom waveguide            cladding region, and n_(Co) is the averaged material            refractive index of the waveguide core region.        -   The electro-active waveguide core thickness d_(CORE) is in            the ultra-thin, very-thin, medium-thin, or thin region such            that d_(CORE)<(2*λ_(op)/n_(Co)).        -   A structure electrically connected to the electro-active            layer comprises at least a first NqN junction in which a            first N-layer with N-dopant is vertically connected            (vertical means in a direction perpendicular to the            substrate plane; horizontal means in a direction parallel to            the substrate plane) to a first q-layer with either N or P            dopant or that is undoped (i.e. being an Intrinsic            semiconductor material) and the first q-layer is further            connected to a second N-layer with N-dopant.        -   A voltage is applied across the first N-layer and the second            N-layer of this first NqN junction to result in an applied            electric field, an electric current, or either injection or            depletion of carriers in the electro-active layer.    -   8. (PqN case plus Tunnel plus TCO) A        lowloss-low-voltage-high-frequency optical phase or intensity        modulator device deposed on a substrate. The device has at least        an input connecting waveguide core deposed on the substrate        connecting the energy of an optical beam to and from an        electro-active layer. The optical beam has one or more optical        wavelengths around an operating optical wavelength λ_(op).        -   The input connecting waveguide core becomes an input            tapering waveguide core and enters below an electro-active            layer. The optical beam energy is well-confined in the input            tapering waveguide core before the tapering waveguide core            enters below the electro-active layer. The optical beam            energy is no longer well-confined in the input tapering            waveguide core at some point after the tapering waveguide            core enters below the electro-active layer.        -   The refractive index n_(EC) or the optical gain/absorption            coefficient α_(EC) of at least part of the material in the            electro-active layer can be altered by an applied electric            field, an electric current, or either injection or depletion            of carriers in the electro-active layer. The electro-active            layer is either part of or in spatial proximity to an            electro-active waveguide core.        -   The electro-active waveguide core and electro-active            waveguide cladding structure is in a medium-strongly guiding            or very-strongly guiding regime such that the refractive            index contrast of the waveguide core layer with both the top            and the bottom waveguide cladding defined by:            R_(cts)=(n_(Co) ²−n_(Cd) ²)/(n_(Co) ²+n_(Cd) ²) are both            larger than about 0.2, where n_(Cd) is the averaged material            refractive index of either the top or the bottom waveguide            cladding region, and n_(Co) is the averaged material            refractive index of the waveguide core region.        -   The electro-active waveguide core thickness d_(CORE) is in            the ultra-thin, very-thin, medium-thin, or thin region such            that d_(CORE)<(2*λ_(op)/n_(Co)).        -   A structure electrically connected to the electro-active            layer comprises at least a first PN junction in which a            first P-layer with P-dopant is vertically connected            (vertical means in a direction perpendicular to the            substrate plane; horizontal means in a direction parallel to            the substrate plane) to a first N-layer with N-dopant, or a            PqN junction in which a first P-layer with P-dopant is            connected to a first q-layer with either N or P dopant or            that is undoped (i.e. being an Intrinsic semiconductor            material) and the q-layer is further connected to a first            N-layer with N-dopant.        -   The first P-layer is electrically connected to a second            P-layer with P-dopant of a second PN junction, referred to            as the PN-changing PN junction (PNCPN). This second P-layer            is electrically connected to a second N-layer with N-dopant            of this second PN junction.        -   A voltage is applied across the second N-layer of the second            PN junction, and the first N-layer of the first PN junction            to result in an applied electric field, an electric current,            or either injection or depletion of carriers in the            electro-active layer.        -   The electro-active layer has a low-refractive-index Ohmic            transparent conductor (LRI-OTC) layer electrically connected            from the top to the electro-active layer. The LRI-OTC forms            part of the top electro-active waveguide cladding.    -   9. (PqN case plus Tunnel plus taper WG) A        lowloss-low-voltage-high-frequency optical phase or intensity        modulator device deposed on a substrate. The device has at least        an input connecting waveguide core deposed on the substrate        connecting the energy of an optical beam to and from an        electro-active layer. The optical beam has one or more optical        wavelengths around an operating optical wavelength λ_(op).        -   The input connecting waveguide core becomes an input            tapering waveguide core and enters below an electro-active            layer. The optical beam energy is well-confined in the input            tapering waveguide core before the tapering waveguide core            enters below the electro-active layer. The optical beam            energy is no longer well-confined in the input tapering            waveguide core at some point after the tapering waveguide            core enters below the electro-active layer. The refractive            index of the tapering waveguide core is given by            n_(ITWCo-z1).        -   The width w_(ITWCo-z1) of the input tapering waveguide core            after penetrating below the electro-active layer is reduced            from a value approximately equal to or larger than half the            wavelength in the material λ_(op)/(2×n_(ITWCo-z1)) to a            value smaller than half the wavelength in the material            λ_(op)/(2×n n_(ITWCo-z1)), so that w_(ITWCo-z1)<λ_(op)/(2×n            n_(ITWCo-z1)) at some point under the electro-active layer.        -   The refractive index n_(EC) or the optical gain/absorption            coefficient □_(EC) of at least part of the material in the            electro-active layer can be altered by an applied electric            field, an electric current, or either injection or depletion            of carriers in the electro-active layer. The electro-active            layer is either part of or in spatial proximity to an            electro-active waveguide core.        -   The electro-active waveguide core and electro-active            waveguide cladding structure is in a medium-strongly guiding            or very-strongly guiding regime such that the refractive            index contrast of the waveguide core layer with both the top            and the bottom waveguide cladding defined by:            R_(cts)=(n_(Co) ²−n_(Cd) ²)/(n_(Co) ²+n_(Cd) ²) are both            larger than about 0.2, where n_(Cd) is the averaged material            refractive index of either the top or the bottom waveguide            cladding region, and n_(Co) is the averaged material            refractive index of the waveguide core region.        -   The electro-active waveguide core thickness d_(CORE) is in            the ultra-thin, very-thin, medium-thin, or thin region such            that d_(CORE)<(2*λ_(op)/n_(Co)).        -   A structure electrically connected to the electro-active            layer comprises at least a first PN junction in which a            first P-layer with P-dopant is vertically connected            (vertical means in a direction perpendicular to the            substrate plane; horizontal means in a direction parallel to            the substrate plane) to a first N-layer with N-dopant, or a            PqN junction in which a first P-layer with P-dopant is            connected to a first q-layer with either N or P dopant or            that is undoped (i.e. being an Intrinsic semiconductor            material) and the q-layer is further connected to a first            N-layer with N-dopant.        -   The first P-layer is electrically connected to a second            P-layer with P-dopant of a second PN junction, referred to            as the PN-changing PN junction (PNCPN). This second P-layer            is electrically connected to a second N-layer with N-dopant            of this second PN junction.        -   A voltage is applied across the second N-layer of the second            PN junction, and the first N-layer of the first PN junction            to result in an applied electric field, an electric current,            or either injection or depletion of carriers in the            electro-active layer.    -   10. (PqN case plus Tunnel plus taper WG plus QW) A        lowloss-low-voltage-high-frequency optical phase or intensity        modulator device deposed on a substrate. The device has at least        an input connecting waveguide core deposed on the substrate        connecting the energy of an optical beam to and from an        electro-active layer. The optical beam has one or more optical        wavelengths around an operating optical wavelength λ_(op).        -   The input connecting waveguide core becomes an input            tapering waveguide core and enters below an electro-active            layer. The optical beam energy is well-confined in the input            tapering waveguide core before the tapering waveguide core            enters below the electro-active layer. The optical beam            energy is no longer well-confined in the input tapering            waveguide core at some point after the tapering waveguide            core enters below the electro-active layer. The refractive            index of the tapering waveguide core is given by            n_(ITWCo-z1).        -   The width w_(ITWCo-z1) of the input tapering waveguide core            after penetrating below the electro-active layer is reduced            from a value approximately equal to or larger than half the            wavelength in the material λ_(op)/(2×n_(ITWCo-z1)) to a            value smaller than half the wavelength in the material            λ_(op)/(2×n n_(ITWCo-z1)), so that w_(ITWCo-z1)<λ_(op)/(2×n            n_(ITWCo-z1)) at some point under the electro-active layer.        -   The refractive index n_(EC) or the optical gain/absorption            coefficient α_(EC) of at least part of the material in the            electro-active layer can be altered by an applied electric            field, an electric current, or either injection or depletion            of carriers in the electro-active layer. The electro-active            layer is either part of or in spatial proximity to an            electro-active waveguide core.        -   The electro-active waveguide core and electro-active            waveguide cladding structure is in a medium-strongly guiding            or very-strongly guiding regime such that the refractive            index contrast of the waveguide core layer with both the top            and the bottom waveguide cladding defined by:            R_(cts)=(n_(Co) ²−n_(Cd) ²)/(n_(Co) ²+n_(Cd) ²) are both            larger than about 0.2, where n_(Cd) is the averaged material            refractive index of either the top or the bottom waveguide            cladding region, and n_(Co) is the averaged material            refractive index of the waveguide core region.        -   The electro-active waveguide core thickness d_(CORE) is in            the ultra-thin, very-thin, medium-thin, or thin region such            that d_(CORE)<(2*λ_(op)/n_(Co)).        -   A structure electrically connected to the electro-active            layer comprises at least a first PN junction in which a            first P-layer with P-dopant is vertically connected            (vertical means in a direction perpendicular to the            substrate plane; horizontal means in a direction parallel to            the substrate plane) to a first N-layer with N-dopant, or a            PqN junction in which a first P-layer with P-dopant is            connected to a first q-layer with either N or P dopant or            that is undoped (i.e. being an Intrinsic semiconductor            material) and the q-layer is further connected to a first            N-layer with N-dopant.        -   The first P-layer is electrically connected to a second            P-layer with P-dopant of a second PN junction, referred to            as the PN-changing PN junction (PNCPN). This second P-layer            is electrically connected to a second N-layer with N-dopant            of this second PN junction.        -   A voltage is applied across the second N-layer of the second            PN junction, and the first N-layer of the first PN junction            to result in an applied electric field, an electric current,            or either injection or depletion of carriers in the            electro-active layer.    -   11. (PqN case plus Tunnel plus taper WG plus doped QW) A        lowloss-low-voltage-high-frequency optical phase or intensity        modulator device deposed on a substrate. The device has at least        an input connecting waveguide core deposed on the substrate        connecting the energy of an optical beam to and from an        electro-active layer. The optical beam has one or more optical        wavelengths around an operating optical wavelength λ_(op).        -   The input connecting waveguide core becomes an input            tapering waveguide core and enters below an electro-active            layer. The optical beam energy is well-confined in the input            tapering waveguide core before the tapering waveguide core            enters below the electro-active layer. The optical beam            energy is no longer well-confined in the input tapering            waveguide core at some point after the tapering waveguide            core enters below the electro-active layer. The refractive            index of the tapering waveguide core is given by            n_(ITWCo-z1).        -   The width w_(ITWCo-z1) of the input tapering waveguide core            after penetrating below the electro-active layer is reduced            from a value approximately equal to or larger than half the            wavelength in the material λ_(op)/(2×n_(ITWCo-z1)) to a            value smaller than half the wavelength in the material            λ_(op)/(2×n n_(ITWCo-z1)), so that w_(ITWCo-z1)<λ_(op)(2×n            n_(ITWCo-z1)) at some point under the electro-active layer.        -   The refractive index n_(CE) or the optical gain/absorption            coefficient α_(EC) of at least part of the material in the            electro-active layer can be altered by an applied electric            field, an electric current, or either injection or depletion            of carriers in the electro-active layer. The electro-active            layer is either part of or in spatial proximity to an            electro-active waveguide core.        -   The electro-active waveguide core and electro-active            waveguide cladding structure is in a medium-strongly guiding            or very-strongly guiding regime such that the refractive            index contrast of the waveguide core layer with both the top            and the bottom waveguide cladding defined by:            R_(cts)=(n_(Co) ²−n_(Cd) ²)/(n_(Co) ²+n_(Cd) ²) are both            larger than about 0.2, where n_(Cd) is the averaged material            refractive index of either the top or the bottom waveguide            cladding region, and n_(Co) is the averaged material            refractive index of the waveguide core region.        -   The electro-active waveguide core thickness d_(CORE) is in            the ultra-thin, very-thin, medium-thin, or thin region such            that d_(CORE)<(2*λ_(op)/n_(Co)).        -   A structure electrically connected to the electro-active            layer comprises at least a first PN junction in which a            first P-layer with P-dopant is vertically connected            (vertical means in a direction perpendicular to the            substrate plane; horizontal means in a direction parallel to            the substrate plane) to a first N-layer with N-dopant, or a            PqN junction in which a first P-layer with P-dopant is            connected to a first q-layer with either N or P dopant or            that is undoped (i.e. being an Intrinsic semiconductor            material) and the q-layer is further connected to a first            N-layer with N-dopant.        -   At least one of the first P-layer, first N-layer, or the            first q-layer contains at least one quantum well. The doping            density at the quantum well is in the highly-doped,            medium-highly-doped, very-highly-doped, or            ultra-highly-doped regime with a dopant density higher than            about 2×10¹⁷/cm³ with either N doping or P doping.        -   The first P-layer is electrically connected to a second            P-layer with P-dopant of a second PN junction, referred to            as the PN-changing PN junction (PNCPN). This second P-layer            is electrically connected to a second N-layer with N-dopant            of this second PN junction.        -   A voltage is applied across the second N-layer of the second            PN junction, and the first N-layer of the first PN junction            to result in an applied electric field, an electric current,            or either injection or depletion of carriers in the            electro-active layer.    -   12. (PqN case plus Tunnel plus taper WG plus very highly doped        QW) A lowloss-low-voltage-high-frequency optical phase or        intensity modulator device deposed on a substrate. The device        has at least an input connecting waveguide core deposed on the        substrate connecting the energy of an optical beam to and from        an electro-active layer. The optical beam has one or more        optical wavelengths around an operating optical wavelength        λ_(op).        -   The input connecting waveguide core becomes an input            tapering waveguide core and enters below an electro-active            layer. The optical beam energy is well-confined in the input            tapering waveguide core before the tapering waveguide core            enters below the electro-active layer. The optical beam            energy is no longer well-confined in the input tapering            waveguide core at some point after the tapering waveguide            core enters below the electro-active layer. The refractive            index of the tapering waveguide core is given by            n_(ITWCo-z1).        -   The width w_(ITWCo-z1) of the input tapering waveguide core            after penetrating below the electro-active layer is reduced            from a value approximately equal to or larger than half the            wavelength in the material λ_(op)/(2×n_(ITWCo-z1)) to a            value smaller than half the wavelength in the material            λ_(op)/(2×n n_(ITWCo-z1)), so that w_(ITWCo-z1)<λ_(op)/(2×n            n_(ITWCo-z1)) at some point under the electro-active layer.        -   The refractive index n_(EC) or the optical gain/absorption            coefficient α_(EC) of at least part of the material in the            electro-active layer can be altered by an applied electric            field, an electric current, or either injection or depletion            of carriers in the electro-active layer. The electro-active            layer is either part of or in spatial proximity to an            electro-active waveguide core.        -   The electro-active waveguide core and electro-active            waveguide cladding structure is in a medium-strongly guiding            or very-strongly guiding regime such that the refractive            index contrast of the waveguide core layer with both the top            and the bottom waveguide cladding defined by:            R_(cts)=(n_(Co) ²−n_(Cd) ²)/(n_(Co) ²+n_(Cd) ²) are both            larger than about 0.2, where n_(Cd) is the averaged material            refractive index of either the top or the bottom waveguide            cladding region, and n_(Co) is the averaged material            refractive index of the waveguide core region.        -   The electro-active waveguide core thickness d_(CORE) is in            the ultra-thin, very-thin, medium-thin, or thin region such            that d_(CORE)<(2*λ_(op)/n_(Co)).        -   A structure electrically connected to the electro-active            layer comprises at least a first PN junction in which a            first P-layer with P-dopant is vertically connected            (vertical means in a direction perpendicular to the            substrate plane; horizontal means in a direction parallel to            the substrate plane) to a first N-layer with N-dopant, or a            PqN junction in which a first P-layer with P-dopant is            connected to a first q-layer with either N or P dopant or            that is undoped (i.e. being an Intrinsic semiconductor            material) and the q-layer is further connected to a first            N-layer with N-dopant.        -   At least one of the first P-layer, first N-layer, or the            first q-layer contains at least one quantum well. The doping            density at the quantum well is in the medium-highly-doped,            very-highly-doped, or ultra-highly-doped regime with a            dopant density higher than about 5×10¹⁷/cm³ with either N            doping or P doping.        -   The first P-layer is electrically connected to a second            P-layer with P-dopant of a second PN junction, referred to            as the PN-changing PN junction (PNCPN). This second P-layer            is electrically connected to a second N-layer with N-dopant            of this second PN junction.        -   A voltage is applied across the second N-layer of the second            PN junction, and the first N-layer of the first PN junction            to result in an applied electric field, an electric current,            or either injection or depletion of carriers in the            electro-active layer.    -   13. (PqN case plus Tunnel plus taper WG plus ultra-highly doped        QW) A lowloss-low-voltage-high-frequency optical phase or        intensity modulator device deposed on a substrate. The device        has at least an input connecting waveguide core deposed on the        substrate connecting the energy of an optical beam to and from        an electro-active layer. The optical beam has one or more        optical wavelengths around an operating optical wavelength        λ_(op).        -   The input connecting waveguide core becomes an input            tapering waveguide core and enters below an electro-active            layer. The optical beam energy is well-confined in the input            tapering waveguide core before the tapering waveguide core            enters below the electro-active layer. The optical beam            energy is no longer well-confined in the input tapering            waveguide core at some point after the tapering waveguide            core enters below the electro-active layer. The refractive            index of the tapering waveguide core is given by            n_(ITWCo-z1).        -   The width w_(ITWCo-z1) of the input tapering waveguide core            after penetrating below the electro-active layer is reduced            from a value approximately equal to or larger than half the            wavelength in the material λ_(op)/(2×n_(ITWCo-z1)) to a            value smaller than half the wavelength in the material            λ_(op)/(2×n n_(ITWCo-z1)), so that w_(ITWCo-z1)<λ_(op/()2×n            n_(ITWCo-z1)) at some point under the electro-active layer.        -   The refractive index n_(EC) or the optical gain/absorption            coefficient α_(EC) of at least part of the material in the            electro-active layer can be altered by an applied electric            field, an electric current, or either injection or depletion            of carriers in the electro-active layer. The electro-active            layer is either part of or in spatial proximity to an            electro-active waveguide core.        -   The electro-active waveguide core and electro-active            waveguide cladding structure is in a medium-strongly guiding            or very-strongly guiding regime such that the refractive            index contrast of the waveguide core layer with both the top            and the bottom waveguide cladding defined by:            R_(cts)=(n_(Co) ²−n_(Co) ²) (n_(Co) ²+n_(Co) ²) are both            larger than about 0.2, where n_(Cd) is the averaged material            refractive index of either the top or the bottom waveguide            cladding region, and n_(Co) is the averaged material            refractive index of the waveguide core region.        -   The electro-active waveguide core thickness d_(CORE) is in            the ultra-thin, very-thin, medium-thin, or thin region such            that d_(CORE)<(2*λ_(op)/n_(Co)).        -   A structure electrically connected to the electro-active            layer comprises at least a first PN junction in which a            first P-layer with P-dopant is vertically connected            (vertical means in a direction perpendicular to the            substrate plane; horizontal means in a direction parallel to            the substrate plane) to a first N-layer with N-dopant, or a            PqN junction in which a first P-layer with P-dopant is            connected to a first q-layer with either N or P dopant or            that is undoped (i.e. being an Intrinsic semiconductor            material) and the q-layer is further connected to a first            N-layer with N-dopant.        -   At least one of the first P-layer, first N-layer, or the            first q-layer contains at least one quantum well. The doping            density at the quantum well is in the very-highly-doped, or            ultra-highly-doped regime with a dopant density higher than            about 1.5×10¹⁸/cm³ with either N doping or P doping.        -   The first P-layer is electrically connected to a second            P-layer with P-dopant of a second PN junction, referred to            as the PN-changing PN junction (PNCPN). This second P-layer            is electrically connected to a second N-layer with N-dopant            of this second PN junction.        -   A voltage is applied across the second N-layer of the second            PN junction, and the first N-layer of the first PN junction            to result in an applied electric field, an electric current,            or either injection or depletion of carriers in the            electro-active layer.    -   14. (PqN case plus Tunnel plus taper WG plus ultra-highly doped        QW plus top side conduction) A low        loss-low-voltage-high-frequency optical phase or intensity        modulator device deposed on a substrate. The device has at least        an input connecting waveguide core deposed on the substrate        connecting the energy of an optical beam to and from an        electro-active layer. The optical beam has one or more optical        wavelengths around an operating optical wavelength λ_(op).        -   The input connecting waveguide core becomes an input            tapering waveguide core and enters below an electro-active            layer. The optical beam energy is well-confined in the input            tapering waveguide core before the tapering waveguide core            enters below the electro-active layer. The optical beam            energy is no longer well-confined in the input tapering            waveguide core at some point after the tapering waveguide            core enters below the electro-active layer. The refractive            index of the tapering waveguide core is given by            n_(ITWCo-z1).        -   The width w_(ITWCo-z1) of the input tapering waveguide core            after penetrating below the electro-active layer is reduced            from a value approximately equal to or larger than half the            wavelength in the material λ_(op)/(2×n_(ITWCo-z1)) to a            value smaller than half the wavelength in the material            λ_(op)/(2×n n_(ITWCo-z1)), so that w_(ITWCo-z1)<λ_(op)(2×n            n_(ITWCo-z1)) at some point under the electro-active layer.        -   The refractive index n_(EC) or the optical gain/absorption            coefficient α_(EC) of at least part of the material in the            electro-active layer can be altered by an applied electric            field, an electric current, or either injection or depletion            of carriers in the electro-active layer. The electro-active            layer is either part of or in spatial proximity to an            electro-active waveguide core.        -   The electro-active waveguide core and electro-active            waveguide cladding structure is in a medium-strongly guiding            or very-strongly guiding regime such that the refractive            index contrast of the waveguide core layer with both the top            and the bottom waveguide cladding defined by:            R_(cts)=(n_(Co) ²−n_(Co) ²)/(n_(Co) ²+n_(Cd) ²) are both            larger than about 0.2, where n_(Cd) is the averaged material            refractive index of either the top or the bottom waveguide            cladding region, and n_(Co) is the averaged material            refractive index of the waveguide core region.        -   The electro-active waveguide core thickness d_(CORE) is in            the ultra-thin, very-thin, medium-thin, or thin region such            that d_(CORE)<(2*λ_(op)/n_(Co)).

A structure electrically connected to the electro-active layer comprisesat least a first PN junction in which a first P-layer with P-dopant isvertically connected (vertical means in a direction perpendicular to thesubstrate plane; horizontal means in a direction parallel to thesubstrate plane) to a first N-layer with N-dopant, or a PqN junction inwhich a first P-layer with P-dopant is connected to a first q-layer witheither N or P dopant or that is undoped (i.e. being an Intrinsicsemiconductor material) and the q-layer is further connected to a firstN-layer with N-dopant.

At least one of the first P-layer, first N-layer, or the first q-layercontains at least one quantum well. The doping density at the quantumwell is in the very-highly-doped, or ultra-highly-doped regime with adopant density higher than about 1.5×10¹⁸/cm³ with either N doping or Pdoping.

The first P-layer is electrically connected to a second P-layer withP-dopant of a second PN junction, referred to as the PN-changing PNjunction (PNCPN). This second P-layer is electrically connected to asecond N-layer with N-dopant of this second PN junction.

1. A lowloss-low-voltage-high-frequency optical phase or intensitymodulator device deposed on a substrate, comprising: an input connectingwaveguide core deposed on the substrate connecting an energy of anoptical beam to and from an electro-active layer, the optical beamhaving one or more optical wavelengths around an operating opticalwavelength λ_(op); the input connecting waveguide core becomes an inputtapering waveguide core and enters below an electro-active layer,wherein the optical beam energy is well-confined in the input taperingwaveguide core before the tapering waveguide core enters below theelectro-active layer, and the optical beam energy is no longerwell-confined in the input tapering waveguide core at some point afterthe tapering waveguide core enters below the electro-active layer; and arefractive index n_(EC) or the optical gain/absorption coefficientα_(EC) of at least part of a material in the electro-active layer can bealtered by an applied electric field, an electric current, or eitherinjection or depletion of carriers in the electro-active layer, whereinthe electro-active layer is either part of or in spatial proximity to anelectro-active waveguide core.
 2. The device as claimed in claim 1,wherein the electro-active waveguide core and an electro-activewaveguide cladding structure is in a medium-strongly guiding orvery-strongly guiding regime such that a refractive index contrast ofthe waveguide core layer with both a top and a bottom waveguide claddingdefined by: R_(cts)=(n_(Co) ²−n_(Cd) ²)/(n_(Co) ²+n_(Cd) ²) are bothlarger than about 0.2, where n_(Cd) is the averaged material refractiveindex of either the top or the bottom waveguide cladding region, andn_(Co) is the averaged material refractive index of the waveguide coreregion.
 3. The device as claimed in claim 1, wherein the electro-activewaveguide core and electro-active waveguide cladding structure is in anvery-strongly guiding regime such that the refractive index contrast ofthe waveguide core layer with both the top and the bottom waveguidecladding defined by: R_(cts)=(n_(Co) ²−n_(Cd) ²)/(n_(Co) ²+n_(Cd) ²) areboth larger than about 0.5, where n_(Cd) is the averaged materialrefractive index of either the top or the bottom waveguide claddingregion, and n_(Co) is the averaged material refractive index of thewaveguide core region.
 4. The device as claimed in claim 1, wherein theelectro-active waveguide core thickness d_(CORE) is in the ultra-thin,very-thin, medium-thin, or thin region such thatd_(CORE)<(2*λ_(op)/n_(Co)).
 5. The device as claimed in claim 1, whereinthe electro-active waveguide core thickness d_(CORE) is in theultra-thin region or very-thin region such thatd_(CORE)<(λ_(op)/n_(Co)).
 6. The device as claimed in claim 1, whereinthe electro-active layer has a low-refractive-index Ohmic transparentconductor (LRI-OTC) layer electrically connected from the top to theelectro-active layer. The LRI-OTC forms part of the top electro-activewaveguide cladding.
 7. The device as claimed in claim 1, furthercomprising A structure electrically connected to the electro-activelayer comprises at least a first PN junction in which a first P-layerwith P-dopant is vertically connected (vertical means in a directionperpendicular to the substrate plane; horizontal means in a directionparallel to the substrate plane) to a first N-layer with N-dopant, or aPqN junction in which a first P-layer with P-dopant is connected to afirst q-layer with either N or P dopant or that is undoped (i.e. beingan Intrinsic semiconductor material) and the q-layer is furtherconnected to a first N-layer with N-dopant.
 8. The device as claimed inclaim 1, further comprising a structure electrically connected to theelectro-active layer comprises at least a first NqN junction in which afirst N-layer with N-dopant is vertically connected (vertical means in adirection perpendicular to the substrate plane; horizontal means in adirection parallel to the substrate plane) to a first q-layer witheither N or P dopant or that is undoped (i.e. being an Intrinsicsemiconductor material) and the first q-layer is further connected to asecond N-layer with N-dopant.
 9. (canceled)
 10. The device as claimed inclaim 1, wherein a voltage is applied across the first P-layer of thisfirst PN junction, and the first N-layer of the first PN junction toresult in an applied electric field, an electric current, or eitherinjection or depletion of carriers in the electro-active layer.
 11. Thedevice as claimed in claim 1, wherein a voltage is applied across thesecond N-layer of the second PN junction, and the first N-layer of thefirst PN junction to result in an applied electric field, an electriccurrent, or either injection or depletion of carriers in theelectro-active layer.
 12. The device as claimed in claim 1, wherein avoltage is applied across the first N-layer and the second N-layer ofthis first NqN junction to result in an applied electric field, anelectric current, or either injection or depletion of carriers in theelectro-active layer.
 13. A lowloss-low-voltage-high-frequency opticalphase or intensity modulator device deposed on a substrate, comprising:an input connecting waveguide core deposed on the substrate connectingan energy of an optical beam to and from an electro-active layer, theoptical beam having one or more optical wavelengths around an operatingoptical wavelength λ_(op); the input connecting waveguide core becomesan input tapering waveguide core and enters below an electro-activelayer, wherein the optical beam energy is well-confined in the inputtapering waveguide core before the tapering waveguide core enters belowthe electro-active layer, and the optical beam energy is no longerwell-confined in the input tapering waveguide core at some point afterthe tapering waveguide core enters below the electro-active layer andthe refractive index of the tapering waveguide core is given byn_(ITWCo-z1); a refractive index n_(EC) or the optical gain/absorptioncoefficient n_(EC) of at least part of a material in the electro-activelayer can be altered by an applied electric field, an electric current,or either injection or depletion of carriers in the electro-activelayer, wherein the electro-active layer is either part of or in spatialproximity to an electro-active waveguide core; and the widthw_(ITWCo-z1) of the input tapering waveguide core after penetratingbelow the electro-active layer is reduced from a value approximatelyequal to or larger than half the wavelength in the materialλ_(op)/(2×n_(ITWCo-z1)) to a value smaller than half the wavelength inthe material λ_(op)/(2×n n_(ITWCo-z1)), so that w_(ITWCo-z1)<λ_(op)/(2×nn_(ITWCo-z1)) at some point under the electro-active layer. wherein theelectro-active waveguide core and electro-active waveguide claddingstructure is in a medium-strongly guiding or very-strongly guidingregime such that the refractive index contrast of the waveguide corelayer with both the top and the bottom waveguide cladding defined by:R_(cts)=(n_(Co) ²−n_(Cd) ²)/(n_(Co) ²+n_(Cd) ²) are both larger thanabout 0.5, where n_(Cd) is the averaged material refractive index ofeither the top or the bottom waveguide cladding region, and n_(Co) isthe averaged material refractive index of the waveguide core region. 14.The device as claimed in claim 13, wherein the electro-active waveguidecore thickness d_(CORE) is in the ultra-thin, very-thin, medium-thin, orthin region such that d_(CORE)<(2*λ_(op)/n_(Co)).
 15. The device asclaimed in claim 13, further comprising a structure electricallyconnected to the electro-active layer comprises at least a first PNjunction in which a first P-layer with P-dopant is vertically connected(vertical means in a direction perpendicular to the substrate plane;horizontal means in a direction parallel to the substrate plane) to afirst N-layer with N-dopant, or a PqN junction in which a first P-layerwith P-dopant is connected to a first q-layer with either N or P dopantor that is undoped (i.e. being an Intrinsic semiconductor material) andthe q-layer is further connected to a first N-layer with N-dopant. 16.(canceled)
 17. The device as claimed in claim 13, wherein a voltage isapplied across the second N-layer of the second PN junction, and thefirst N-layer of the first PN junction to result in an applied electricfield, an electric current, or either injection or depletion of carriersin the electro-active layer.
 18. The device as claimed in claim 13,wherein at least one of the first P-layer, first N-layer, or the firstq-layer contains at least one quantum well. The doping density at thequantum well is in the highly-doped, medium-highly-doped,very-highly-doped, or ultra-highly-doped regime with a dopant densityhigher than about 2×10¹⁷/cm³ with either N doping or P doping.
 19. Thedevice as claimed in claim 13, wherein at least one of the firstP-layer, first N-layer, or the first q-layer contains at least onequantum well. The doping density at the quantum well is in themedium-highly-doped, very-highly-doped, or ultra-highly-doped regimewith a dopant density higher than about 5×10¹⁷/cm³ with either N dopingor P doping.
 20. The device as claimed in claim 13, wherein at least oneof the first P-layer, first N-layer, or the first q-layer contains atleast one quantum well. The doping density at the quantum well is in thevery-highly-doped, or ultra-highly-doped regime with a dopant densityhigher than about 1.5×10¹⁸/cm³ with either N doping or P doping.